Dissected so as to show its motor, digestive, and reproductive systems.
Plate XI. Common Star-Fish, Asterias rubens. LInn.
The dorsal integument has been removed from the central ray of the trivium, I; from its left ray, II; and from both rays of the bivium, a part of it being left attached to the right ray, V, of the bivium at its apex, to show the attachment of the radial digestive coeca. The digestive and other viscera have been removed in great part from the interior of the two rays of the bivium and the ampullae d2, and the ambulacral ossicles k 2 exposed in situ. The coeca attached to the pyloric portion of the stomach have been displaced from their attachments in the left ray of the trivium; they have been left undisturbed in the central ray; and in the right ray all the organs, with the exception of a small part of the dorsal integument next to the central disc, have been left undisturbed.
I. Central radius of trivium; a line drawn along the axis of this ray to the madreporic tubercle gt would, in the undisturbed condition of the parts, have the anus a little on its left; and if prolonged, would pass down the interradial space of the bivium (IV and V).
II. Right radius of trivium, with the greater part of the integument left in situ. The inward prolongation or angle of the integument is well seen in the interradial space between arm I and arm II. The dorsal integument contains a large number of ossicula, some of which carry small conical prickly spines, whilst others simply connect the spini-gerous ossicles into a reticulation. Down the centre of each ray the spinigerous tubercles are in this species arranged with considerable regularity, so as to form a mesial series; in the other portions of the dorsal area, they are scattered irregularly. The intervals between the dorsal ossicula bear respiratory coecal processes of the integument.
III. Left radius of trivium. The two coeca of the pyloric division of the stomach have been displaced from their normal connections within the cavity of the ray, and are displayed in the interradial space on either side. In the middle line are seen the ambulacral ossicles, and on either side the generative glands,j1, ,j2.
IV and V. Left and right rays of bivium. The greater part of the reproductive organs and the whole of the digestive have been removed; and the ambulacral ampullae, d 1, d 2, corresponding to the rows of sucker-like feet arranged on either side of the ventrally placed ambu-lacral furrows, are seen in two rows on either side of the middle line occupied by the mesial articulations of the successive pairs of 'vertebral' or 'ambulacral' ossicles, k 1, k 2.
If the results of Ludwig's researches on the development of Asterina gibbosa are applicable to all Asteroidea, a Starfish ought to be placed as follows to make it coincide with the position of the parts relative to the antero-posterior axis of the larva. The madreporite g must be placed to the left; and the anus e (figured too near the centre of the disc), with the interradius between III and IV, to which it belongs, must be anterior. This interradius coincides on the ventral surface of the adult with the position of the anterior appendage of the larva. Five processes develope on the right side of the larva, i. e. the dorsal or abactinal surface in the adult, and are arranged in a circle, open anteriorly. To form the arms they fuse with five similarly arranged processes of the left side, i. e. the ventral or actinal surface here. Both sets of processes may be numbered from 1-5, commencing with the anterior ventral process of the larva, and ending with the anterior dorsal, passing round the posterior extremity. In the fusion of the two sets of processes, abactinal and actinal, a curious twist of the one set upon the other takes place.
The actinal process 1 coincides with III in the Plate, 3, which marks the posterior extremity of the larva, with II, and 5 with IV. The abactinal processes, which correspond at their first appearance with the actinal, shift so that 2 coincides with III, 4 with II, and 1 with IV. Consequently a line drawn through the anal interradius down arm II marks a line in the adult which coincides with the antero-posterior axis of the larva. The original antero-posterior axis of the abactinal surface passes, however, through the madreporite and arm I; for while this axis retains its position with reference to the larval antero-posterior axis on the actinal surface, it is shifted to the left on the abactinal. It may be noted that the larval anus or gastrula mouth lies between the actinal and abactinal processes 3, but closes before the twist takes place. The larval mouth lies similarly between the two processes 1, but closes, and a new mouth is formed in connection with a new oesophagus which grows out towards the left side of the larva.
It would be interesting to know whether the twist occurs in other Asteroidea, and in Ophiuroidea as well. See Ludwig, Z. W. Z. xxxvii. 1882.
a. Pyloric division of stomach, communicating freely with the cardiac, and giving off a stem which bifurcates as it enters each ray. b 1. One of the arborescent divisions into which the radial diverticulum of radius I divides. It is only in the Asteroidea that the digestive tract has this radial arrangement of coeca. b 2. Arborescent coecum of radius III, displaced, as is its fellow, into the interradial spaces. b 3 and b 4. Terminations of coeca of radius V attached to the dorsal integumentary skeleton by a mesentery. c. Cardiac division of stomach, bulging, but only for a short distance, into the cavity of the several rays at a lower level than the coeca, b. To the right of c is seen one of the interradial septa to which the ducts of the generative coeca on either side are attached. d 1. Ampullae of ambulacral feet of radius IV. d2. Ampullae of radius V.
e. Subcentrally placed anus.
f. Origin of extensor muscle of radius I from inner surface of centre of dorsal integument. It is by the action of this muscle that the distal extremity of the rays and the eyes they carry, have their ordinary up-turned direction, as shown in this figure, given to them. f1. Distal termination of extensor muscle of radius V.
g. Madreporic canal and plate displaced backwards into the interradial space of the bivium, opposite to which it is placed in the natural position of the parts. h. First ambulacral ampulla. j1 andj2. Reproductive glands of radius III, consisting of multiramified coeca appended to a single efferent duct, as in many but not all proctuchous Asteroidea, and in Ctenodiscus amongst the aproctous. j3. Point of attachment of efferent generative duct of right generative gland of radius V to interradial septum, which is formed by the prolongation inwards of the integument containing a number of small flat ossicles. k 1 and k 2. Median ends of ambulacral ossicles forming the vertebral ridge with median furrow.