126 LECTURE X. 



the Holothuria : its anal termination dilates into a cloaca, from which 

 two long ramified caeca are continued ; but these admit only sea 

 w^ater from the cloaca. The alimentary canal in the Sipmiculus* 

 differs from that in the Holothuria in being reflected from the pos- 

 terior 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 Sipiincuhts is a marine vermiform animal w^hich burrows in sand, 

 and, although it has no tegumentary tubular feet nor organs of re- 

 spiration, is most closely allied to the Holothuria^ and is therefore 

 retained in the class Echinoderma, in w^hich it makes the nearest 

 approach to the true Vermes. The anterior position of the vent in the 

 Sipunculus precludes the necessity of the worm quitting the retreat, 

 which its safety demands on account of its integument 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.f Here, however, we 

 find the intestinal vessels carrying the nutrient fluid to those cloacal 

 caeca which are transformed into a distinct respiratory organ, and 

 which presents the form of two long and beautifully arborescent 

 tubes. 



The complex circulating system in the Holothuria is in great part 

 represented in this diagram in connection with the equally extensive 

 system of sinuses and canals which regulate the protrusion and re- 

 traction of the numerous tubular feet. 



The ampulla Poliana {Jig. 67. a), w^hich is double in some species, 

 is the analogue of the blind sacculi, which supply the canal of the 

 bases of the feet in the Asterias, but is called the heart by Delia 

 Chiaje. It transmits its fluid principally to an annular reservoir 

 round the pharynx (6), whence proceed the canals of the oral ten- 

 tacula (c) and those supplying the tubes which perforate the coriaceous 

 integument. The latter canals (c?, d) run down in the interspaces of 

 the pairs of muscles, and distribute transverse branches to the bases 

 of the tubes as they proceed. The most important part of the un- 

 equivocal circulating system is the trunk (e), which runs along the 

 free border of the intestine, and which is characterised by the short 

 and wide anastomotic trunk (/, /) analogous to the heart in the 

 Echinus, and which connects the corresponding vessels of the two 

 principal folds of the intestine. The intestinal capillaries reunite^ 



* Prep. No. 438. A. 



f See Preps. Nos. 437, 438. 984. 



