Anatomy of Holothuria: a, bristle inserted into the mouth; b, inverted tentacula.

Fig. 101. Anatomy of Holothuria: a, bristle inserted into the mouth; b, inverted tentacula; c, ampulla Poliana; ddd, intestinal canal; e, cloacal chamber opening externally by a wide orifice, into which the bristle f, has been passed; gg, "respiratory tree;" hh, ovarium (testis in the male); ii, central vascular trunk; k, intestinal vessel; l,m, vessels in relation with the "respiratory tree".

(537). The circulation of the blood in the Holothuria, as in the Echinus, is still but imperfectly understood; and considerable difference of opinion upon this subject will be found in the writings of anatomists. According to Tiedemann*, innumerable small veins collect the blood and nutritive products of digestion from the intestine and convey them into a large central vessel (fig. 101, ii), from whence the circulating fluid passes by other trunks (I I) to the respiratory tree; hence it is returned by vessels (partly represented at m) to the intestinal artery (7c), by which it is again distributed over the intestinal parietes.

(538). Delle Chiaje gives a different account of the arrangement of the vascular system in these creatures, which he seems to have investigated with his usual untiring perseverance. According to the last-mentioned anatomist, the blood is taken up from the intestines by a complicated system of veins, the main trunks of which are indicated in the annexed diagram (fig. 102) by the letters c, e,pp, q q; these communicate with each other not only by the intervention of numerous anastomosing branches (d d), but likewise by means of delicate vascular plexuses (a) passing between them. All these veins terminate in two large venous canals (o) that convey the blood and nutriment absorbed from the intestine to a vascular circle (g) placed around the commencement of the oesophagus, which corresponds with the circular vessel around the mouth of the Echinus. This circle Delle Chiaje regards as the centre of the arterial system, in communication with which is the contractile vesicle (f); and this he looks upon as a reservoir for the nutritive fluid. From the circular vessel various arteries are given off: large branches pass into the tentacula around the mouth (i); so that these organs, besides being instruments of touch, from the extent of surface that they present and their great vascularity, are most probably important auxiliaries in respiration. Five other large arteries, derived from the same source (k k, I), pass backwards to supply the integuments of the body, and also to communicate by small cross branches with the little vesicular organs connected with the locomotive suckers, which, in the opinion of Delle Chiaje, are distended with the same blood as that which circulates through the rest of the body.

The descending arteries, thus destined to supply the integument and distend the prehensile suckers, run in the centre of each of the five longitudinal fasciculi of the muscular tunic of the skin as far as the cloaca, and exhibit in their distribution a remarkable exception to the usual arrangement of the arterial system, which is generally found to divide and subdivide continually into smaller and still smaller canals: in the case before us there would seem to be no diminution in the size of the main trunks as they approach their termination; and the cross branches given off in their course, instead of ramifying, all end in the minute ambulacral vesicles, to the injection of which they would appear to be subservient.

Plan of the circulation in Holothuria, according to Delle Chiaje.

Fig. 102. Plan of the circulation in Holothuria, according to Delle Chiaje.

* Anat. der Rohren-Holothurie. Fol 1816.

Embryology of Holothuria.

Fig. 103. Embryology of Holothuria.

(539). The generative system of the Holothuria is essentially similar to that found in the Asteridae, consisting of long ovigerous caeca. The germs are secreted in slender ramified tubes (fig. 101, h h); these are collected into one great bundle, and open externally by a common canal in the neighbourhood of the mouth - not into the oesophagus, as Cuvier supposed, but upon the back of the animal. The generative caeca at certain times of the year become enormously distended, being at least thirty times as large as when not in a gravid state: if examined at this period, they are found to contain a whitish, yellowish, or reddish fluid, in which, in the female, the ova are suspended. In the male a precisely similar structure exists; but instead of ova, the caeca contain a fluid crowded with spermatozoa during the breeding season.

(540). After their escape from the egg, the young Holothuriae have been ascertained to undergo a kind of metamorphosis scarcely less wonderful than that observed in Ophiura and Echinus. In its first or Pluteus condition, the little embryo bears no resemblance whatever to the future animal, but swims vigorously about by the agency of broad membranous-looking expansions that surround the margins of its flattened body, wherein the stomach and other viscera are distinguishable (fig. 103, 1, 2.) In its second stage of existence it has somewhat the appearance of a polype (fig. 103, 3); and this ultimately becomes converted into a larva-like being (fig. 103, 4), surrounded with several rows of vibratile cilia, by means of which its progression is accomplished. In the interior of this larva, a set of rudimentary oral tentacula, surrounded at their bases by a circle of calcareous spicula, is developed, an alimentary canal makes its appearance, and even the ampullae Polianae are distinctly recognizable, surrounding the position of the future mouth. In its fourth stage of advancement (fig. 104), the Ho-lothurian structure is no longer doubtful, although the apparatus of vibratile cilia still exists upon the exterior of the body. The alimentary canal (a) may be seen to terminate in a cloacal chamber, the Polian vesicle (6) is largely increased in size, the calcareous circle (c) around the mouth is much strengthened, the tentacles (d)have assumed larger proportions, and even the appearance of the suctorial feet (e) is no longer doubtful; the longitudinal muscular fasciculi in the integument progressively acquire strength, and the little creature is transformed into the shape and attains the proportions of its parent.