516 



COMPARATIVE ANATOMY 



CHAP. 



out in front of them (Fig. 415). The first two tube-feet in Cucumaria Planci arise 

 simultaneously at the posterior end of the body. Both these feet belong to the 

 medioventral radial canal. 



The differentiation of the enteroccel vesicle. After the hydro-enteroca-1 

 vesicle has divided into the hydrocoel and the enteroccel vesicles, the latter grows 

 backwards longitudinally, the growing posterior end pushing its way gradually over 

 the intestine, on to its right side (i.e. into the right-hand portion of the segmenta- 

 tion cavity). The anterior part of the enterocoel vesicle now lies to the left, the 



posterior to the right, near the intestine (Fig. 

 404). These parts become completely separated 

 by a constriction which stretches transversely over 

 the intestine, into a left and a right enteroccel 

 vesicle. 



Each of these vesicles becomes applied to the 

 intestine, and, increasing in size, takes the shape 

 of a hollow disc resembling a watch glass. 



The nervous system of the larva. On each 

 side of the Auricularia larva, on the ventral side, 

 in the oral area, there is a ciliated ectodermal 

 ridge, beneath the surface of which ganglion cells 

 lie and longitudinal nerve fibres run. The ridge 

 consists of two limbs, meeting in an obtuse angle 

 which is open towards the mouth. From the ends 

 and angles of these ridges nerve fibres go off to 

 the circumoral ciliated band. 



Formation of the tentacles. The tentacle 



vessels of the water vascular system ; 

 7, hind -gut; 8, calcareous wheels; 

 9, mid-gut ; 10, madreporite ; 11, stone 

 canal. 



FIG. 414. Young Synapta (Pentac- 

 tula) (after Semon). 1, Oral tentacles ; 

 2, auditory vesicles ; 3, pieces of the 

 calcareous ring ; 4, water vascular 

 ring ; 5, Poliau vesicle ; 6, radial canals, whether as lateral outgrowths of the radial 



canals or direct outgrowths of the water vascular 

 system, grow towards the oral vestibule, and press 

 out the ectodermal wall of the latter before them. 

 The ectodermal covering thus afforded them is 

 derived from the oral shield, i.e. indirectly from parts of the original circumoral 

 ciliated ring of the Auricularia larva. The tentacles (five of which form first) 

 remain hidden in the oral vestibule during the pupal stage. 



Transformation of the barrel- shaped larva into the young Holothurian (Figs. 

 414 and 415). The external changes are as follows. The ciliated rings atrophy. 

 The tentacles project freely from the expanding and widely opening vestibule, and 

 increase in number. In the Actinopoda tube-feet are formed in all the five radii. 



It is an important fact that, in the comparatively simple transformations of the 

 Holothurian, not only does the whole of the larval epithelium pass into the body 

 epithelium of the adult, but none of the larval organs are eliminated. 



The following description applies to Cucumaria Planci. 



The tentacles. It is somewhat remarkable that the five tentacles first formed do 

 not each arise from a single radial canal. Two of the five tentacles, on the contrary, 

 receive their canals from the medioventral, and two others from the left dorsal 

 radial canal. The fifth tentacle belongs to the right dorsal radial canal. Other 

 tentacles appear only at a very late stage, two more being added first, a sixth and a 

 seventh. These belong to the two lateral ventral radii, which up to this time have 

 been without tentacles. 



The stone canal. An anterior outgrowth forms on the primary stone canal ; the 

 epithelium of this outgrowth flattens, giving rise to the madreporitic vesicle. On 

 the wall of this vesicle the mesenchyme forms an incomplete shell, perforated like a 

 lattice 



