20 Echinodernia. 



of tlie hind-gut with more or less folded walls, and this is probably the ancestral 

 form of the paired gills. One lies in the left ventral and the other in the right 

 dorsal interradius in the spaces left free by the gut and mesentery, and they 

 usually have a common opening into the cloaca, though sometimes separated in 

 the Molpadiidae and Dendrochirotae. The supposed presence of additional gills 

 in these families is simply due to division of the stem of the left gill near its base. 

 L. has never succeeded in finding the openings of the gills into the coelom which 

 were described by Semper and Hamanu. 



Herouard gives a description of the anatomy of Dendrochirotae, which is chiefly 

 based upon observations on Cucumaria Planet, Colochirus Lacazii (n. sp.) and 

 Holothuria catanensis. The smaller plates of the calcareous armature , without 

 nodosities, are situated in the epidermis, immediately beneath the cuticle. Under 

 this is the thick connective tissue of the dermis, with the larger and more nodular 

 plates. Between this and the muscular layer is the middle zone, the outer part 

 of which contains the wander cells and belongs to the lacunar system, while its 

 deeper portion contains abundant nerve fibres and may therefore be called the 

 nervous layer. - The parts of the fundamental calcareous corpuscle, 

 which is formed along the apposed edges of four hexagonal cells, remain constant 

 in their form, relative dimensions, and in the asperities which they present. The 

 corpuscles of the young animal are often more complex than those of the adult, 

 e.g. in Colochirus Lacazii and Thy one subvillosa. The perianal teeth of the latter 

 type belong to the internal zone of the body wall, and, being radial in position, 

 are homologous with the ocular plates of the Urchins. The pieces of the oral 

 calcareous ring also belong to the internal zone, and the forked summits of the 

 radialia correspond to the auricles of Urchins. - The internal epithelium of 

 the water- vascular system, like that of the body cavity, is only attached 

 to the subjacent connective tissue by filiform processes, so as to leave a subepi- 

 thelial lacuna. In the water -vessels this contains the muscular fibres. The epi- 

 thelium of the Polian vesicle has plastidogenic functions, its cells readily becoming 

 detached. Each radial water-vessel consists of three parts, ambulacral, coronal, 

 and the dilated basal portion. The tube-feet arise from the ambulacral part, their 

 openings being guarded by valvules. The coronal part, situated between a radial 

 piece of the calcareous ring and the peripharyngeal sinus, is marked in Cucumaria 

 by the two openings of the tentacular water-vessels which are also guarded by 

 valves. The tentacles are in reality modified tube-feet, adapted to special func- 

 tions ; and like the ordinary tube-feet consist of two parts , an external, or 

 tentacle proper, and an internal ampullar portion, the cul de sac tentaeu- 

 laire. The latter has no longitudinal musculature, and is situated between the 

 upper edges of two adjacent pieces of the calcareous ring, while in Holothuria it 

 hangs freely in the body cavity. In Thyone it is internal and covered by the cal- 

 careous ring. The boundary between it and the tentacle is indicated by the val- 

 vular opening of the short lateral branch from the radial water- vessel. The 

 coronal part of the latter in Holothuria gives off four tentacular branches, the 

 proximal pair corresponding to those of Cucumaria, while the distal pair represent 

 the vessels supplying the first pair of tube-feet in that type. The p e r i p h a ry n- 

 geal sinus which is situated between the radial vessels and the fore-gut, is a 

 restricted portion of the coelom, communicating with it by five interradial open- 

 ings. It is traversed by tracts of lacunar tissue, which belong to the middle zone 

 of the body wall, and are in relation with the radial blood-lacuna, lying on the 

 water-vessel. - - The Holothurians having no skeleton to present fixed points for 

 muscular attachment, a sufficient rigidity is produced by means of cavities con- 

 taining liquid, the muscular walls of which can exert pressure on their contents. 



