ECHINODERMATA. 209 



partly defends the nervous ring, and affords a firm attachment 

 to the branched retractile tentacles -which surround the mouth. 

 These tentacles may be likened to a more complicated form of the 

 ordinary tube-feet of the body, each being connected at its base with 

 a long hollow sacculus, and being distended and protruded by the 

 injection of the fluid therein contained. In Synapta the oral circle 

 includes twelve plates, five of vyhich are perforated for the passage 

 of the chylaqueous canals. The integument in this genus is beset 

 with small anchor-shaped spiculae, which are articulated to reticulate 

 calcareous plates in the skin.* 



The alimentary canal of the Holothuria is closely analogous to 

 that of the Echinus ; but its disposition is accommodated to the 

 vermiform character of the softer-skinned species : its anal termina- 

 tion dilates into a cloaca, from which two long ramified caeca are 

 continued ; but these admit only sea-water from the cloaca. In the 

 Synapta {Holothuria digitata, Montagu) the stomachal portion of the 

 alimentary canal is defined by its strong muscular parietes : there 

 are distinct layers of muscular fibres in certain parts of the 

 mesentery, which also has many peculiar slipper-shaped ciliary 

 organs -^^i\i of a line in length, its general surface not being 

 ciliated. 



The alimentary canal in the Sipunculus\ differs from that in 

 the Holothuria in being reflected from the posterior extremity of 

 the body to terminate near the anterior end, without dilating into 

 a cloaca, and without the development of any anal caeca. The 

 intestine is longer and more convoluted in its course. The Sipun- 

 culus is a marine vermiform animal which burrows in sand, and, 

 although it has no tegumentary tube-feet nor organs of respira- 

 tion, is most closely allied to the Holothuria, and is therefore 

 retained in the class Echinoderma, in which it makes the nearest 

 approach to the true Vermes. The oral tentacula are replaced by a 

 kind of proboscis provided with a circular fimbriated lip, and two 

 contractile vesicles appear to be connected with the erection of the 

 fimbriae. The anterior position of the vent in the Sipunculus pre- 

 cludes the necessity of the worm quitting its retreat to evacuate : and 

 its safety seems to demand such a modification on account of its in- 

 tegument being less thick and coriaceous than in the Holothuria. 



The rich vascular system of the Holothuria is most conspicuous 

 upon the intestine and mesentery, and has been beautifully illustrated 

 by the injections and drawings of Hunter. J Here, however, we 

 find the intestinal vessels carrying^the nutrient fluid to those cloacal 



* CLXVIIL Heft. ii. p. 12. CLXrX. p. 35. f I'rep. No. 438. A. 



X See Preps. Nos. 437, 438. 984., and X. pis. 3. and 49. 



P 



