Dissected so as to show its digestive and motor systems.
ONE of the rays, the central ray of the trivium, has been cut short, and more or less of the dorsal or anti-ambulacral integument removed from each of the other four, and from the central disc. In the interradial space which is opposite to the ray cut short, is seen the madreporite; and a little to the left of a line drawn along the axis of the central ray to the madreporic tubercle, and near the centre of the disc is seen the small piece of dorsal integument in which the anus opens. It lies on this aspect in the interradius, to the left of the madreporite (see note, Plate xi.). From the intestine, and close to the anus, arise two diverticula. They bear several irregular caecal ampullae and reach a short way into two interradii, the interradius between the central and left ray of the trivium and the interradius between the left rays of the bivium and trivium respectively. The internal surface of these diverticula is longitudinally plicated and they are probably highly extensile. They are generally considered to be the homologues of the respiratory trees of the Holothuroidea. The intestine itself cannot be seen. It is short and arises from the pyloric division of the stomach.
This pyloric division is pentagonal in outline, and a single trunk may be seen to arise above each angle of the pentagon. Each trunk enters the corresponding ray, and divides into two branches which, with their foliaceous glandular ampullae, fill up the greater part of the cavity of the ray. The saccular dilatations of the anterior or cardiac division of the stomach are to be seen lying below the trunks of origin of the complex caeca, and bulging for a short distance into the cavities of the rays. These sacculi can be evaginated so as to enclose the animals on which the Starfish feeds, e.g. young oysters, cockles, etc, too large to be drawn into the disc. Each sacculus is retracted after protrusion by a pair of muscles attached to the sides of the vertebral ridge of the ambulacral groove. The Asteroidea are the only group of Echino-dermata which possess a radial development of caeca to the digestive tract as seen here. The two divisions of the arborescent caeca have been separated in two of the rays to show the ampullae of the feet.
These are arranged in two symmetrical rows on either side of the vertebral ridge of the ambulacral ossicles, thus corresponding to the arrangement of the pores seen in the preceding preparation.
On the ventral surface the mouth is to be seen placed centrally. The ambulacral feet are variously contracted, some more, some less. They have sucker-like ends, which are not supported by calcareous plates as they are in Echinoidea and most Holothuroidea. In the left ray of the trivium the feet are completely retracted, and in the left ray of the bivium the series of adambulacral moveable spines has closed completely over the groove, thus protecting the soft parts which it lodges.
The pedicellariae may be seen strewn among the spines of the perisoma, and upon the moveable adambulacral spines. It is possible that some of the minute elevations among the spines are the incompletely retracted tubular respiratory processes of the integument.
The oesophagus is short and longitudinally plicated: the cardiac and pyloric divisions of the stomach are partially separated by a circular fold, and the intestine has a narrow plicated aperture into the stomach. The epithelium of the stomach is stated by Hoffmann to be ciliated.
The arborescent caeca in the arms with their ducts are suspended to the dorsal perisoma by a couple of mesenteric bands. The cells in the caeca form entero-chlorophyll, and tryptic, peptic, and diastatic ferments. They pour their secretion into the stomach.
The development of the genital glands is periodical: when sexually mature they reach far down into the arms. They are branched glands alike in both sexes, and are surrounded by a blood sinus. Each gland has a single duct in A. rubens, but this duct opens by a sieve plate, i. e. a plate pierced by many pores. There are ten plates corresponding to the number of the glands. They are placed inter-radially and dorsally close to the bases of the arms. In most instances the genital plate has only one aperture. These calcareous genital plates have been supposed to be the homologues of the genital, i.e. basal plates of the apical system in Echinoidea. It is certain however that this is not the case: and in Starfish in which the apical system is retained in its typical form, no relation is observable between the genital apertures and the basals. Moreover in certain Starfish there is a more or less numerous series of genital glands, each with its own aperture, extending up the sides of the arms to a greater or less degree. In Asterina gibbosa the genital apertures are ventral. The duct in Asterina pentagona has unicellular glands by which the coat of the ovum is formed. These are probably present in other Starfish as well.
Impregnation is external.
General Anatomy of soft parts. Ludwig, Z. W. Z. xxx. 1878; cf. Carpenter, Q. J. M. xxi. 1881. Add on generative organs. Ludwig, on Asterina gibbosat Z. W. Z. xxxi. 1878.
General minute Anatomy. Beitrage zur Histologic der Echinodermen, Hamann, pt. ii. Die Asteriden, Jena, 1885.
Digestive ferments. Krukenberg, Vergleich. Physiol. Vortrage, ii. 1882, and lit. cited, p. 78. On colouring matters. Id. op. cit. iii. 1884, and lit. cited, p. 179. Enterochlorophyll. MacMunn, P. R. S. xxxv. 1883; Id. P. R. S. xxxviii. 1885.