262 PHYLUM ECHINODERMATA. 



channels. The vascular trunks though called vessels are 

 nothing more than a system of communicating spaces or bundles 

 of anastomosing tubes. They are without any epithelial lining, 

 and contain a coagulable fluid. 



Reproductive organs. The Holothurians are for the most part 

 dioecious, but a few of the Synaptidae and Molpadiidae are 

 hermaphrodite (ova and spermatozoa arising in the same tubes). 

 The gonad is single and lies in the adult-dorsal interradius 

 (Fig. 181). It consists of a tuft of branched tubes projecting 

 into the body-cavity, on both sides of the mesentery (on the 

 left side only in Holothuria, M idler ia, Labidodemas and some 

 Elasipodidae). The genital duct lies in the dorsal mesentery 

 and opens to the exterior in the middle, adult-dorsal line in the 

 anterior region of the body. In the Dendrochirotae it is between 

 two of the tentacles or even within the tentacular circlet ; in the 

 Molpadiidae and Synaptidae it is immediately behind the ten- 

 tacular circlet ; it is furthest removed from the tentacles in the 

 Elasipodidae (in Psycliropotes longicauda it lies in the posterior 

 half of the body). The genital duct is always single, but in some 

 Elasipodidae it divides so as to open by several apertures (in 

 some species the number varies in different individuals). The 

 genital opening may be at the end of a small papilla. The 

 genital rachis, if present, is represented by a cord near the 

 genital duct (p. 132). 



As a rule there are no external sexual differences, but the sexes 

 can sometimes be distinguished by inspection of the generative 

 organs. In a few species the male alone possesses a genital 

 papilla (Thyone aurantiaca, Cucumaria laevigata, etc.). In the 

 connective tissue layer of the wall of the gonad, which lies next 

 the inner epithelium, there is an extensive development of the 

 lacunar spaces of the blood-system. 



They are all marine, and with the exception of one pelagic 

 form (Pelagothuria) they live on the sea bottom, usually attached 

 to external objects by their tube-feet. Most of them are able to 

 crawl by means of their tube-feet, though many of them move 

 but little. Many of those which have been observed are said to 

 be more active during night than in the day time, but no visual 

 organs are known. They are found in all seas and at all depths, 

 many being littoral and a considerable number abyssal. The 

 Elasipodidae are almost entirely deep-sea forms. The Synap- 



