Characteristics of Holothurividea. cxllx 



to the water- vascular ring. The first segment of the digestive tract 

 is suspended by a muscular mesentery along the middle line of the 

 dorsal l)ivium, which is thus made actually as well as morphologi- 

 cally a line for the bilateral division of the body. The terminal 

 segment has, as in so many Invertebrata, a respiratory function, 

 and, in the great majority of Holothurioidea, has certain multi- 

 ramified coeca, the . so-called ' lungs,^ or ' respiratory trees ^ of the 

 ' Pneumonophorous' order, appended to it. Where these ' trees' are 

 absent, as in Synaptidae, certain ciliated funnel-shaped organs are 

 to be found bestudding the mesentery ; and as these organs are in 

 the Gephyrean Worms similarly absent, or present, accordingly 

 as in them cloacal respiratory trees are present {EcMuridae), or 

 absent {Sijmncididae), the two sets of organs may be supposed to 

 stand to each other in a supplementary relation. The apical termi- 

 nations of the respiratory trees are supposed to be perforated, and 

 thus to furnish a route whereby the sea-water can find its way 

 into the perivisceral cavity. As the ciliated infundibula are, accord- 

 ing to the figures of them given by Sars, Norges Echinodermer, 

 Tafs. XV. xvi. ; and Leydig, Lehrbuch der Histologic, p. 391, fig. 

 203; Miiller's Arch., 1852, p. 514; connected with a system of 

 vessels in the mesenteric membrane, it would appear that they may 

 discharge the same function. 



To the respiratory trees or to the cloaca whence they arise, 

 the so-called ' Cuvierian organs ' are appended, which may have a 

 glandular function, but which are probably organs of defence, as 

 they are observed to be very readily discharged upon irritation. 



The pseudhaemal system consists of two main stems, connected 

 the one with the dorsal, and the other with the ventral line of the 

 digestive tube; and of a circular plexus representing the circular 

 pseudhaemal vessel surrounding the pharynx. These two vessels 

 are connected with each other by reticulations in the walls of the 

 intestine, and the ^rete mirabile' developed by the dorsal vessel is 

 brought in many cases {Aspidochirotae) into intimate connection 

 with the left respiratory tree. 



The nervous system is said to differ from the nerve-systems of 

 other Echinodermata by having its circular commissural collar 

 thicker than the radial ' Ambulacral-gehirne.' It appears, however, to 

 be of less complex structure, and it must be recollected that in some 

 other Echinodermata the radial stems taper towards their proximal 



