516 



CO MP A RA TI VE A NA TOM Y 



out in front of them (Fig. 415). The first two tiibc-fcet in Cucxmuiria Planci arise 

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

 medioventral radial canal. 



The diflFerentiation of the enterocoel vesicle. — After the hydro-enterocad 

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

 backwards longitudinally, the growing jiosterior 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 enteroccel vesicle now lies to the left, the 



posterior to the right, near the intestine (Fig. 

 404). These parts become completely separated 

 l)y a constriction which stretches transversely over 

 the intestine, into a left and a right enteroccel 

 vesicle. 



Each of these vesicles becomes apidieil to the 

 intestine, and, increasing in size, takes the shajie 

 of a hollow disc resembling a watch glass. 



The nervous system of the larva. — On each 



side of the Ainicuhi.ria. larva, on tlie 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 



P"i(i. 414.— Young Synapta (Pentac- and angles of these ridges nerve tilires go off to 



tula) (after Semon). 1, Uraltentacles ; the circumoral ciliated band. 



2, auditory vesicle.s ; 3, piece.s of the „ x- r xi. j. j. i rri ^ . i 



, . , ^ , Formation of the tentacles. — ihe tentacle 



calcareous ruig ; 4, water vascular 



rino- • 5 Poliau vesicle ■ 6 radial canals, whether as lateral outgrowths of the radial 

 vessels of the water vascular system ; 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 tlie oral vestibule during the pu)ial stage. 



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

 414 and 415). — Tlie 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 Adiiiopoda 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 jiass into the body 

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



The following description applies to C'ueuinaria Planci. 



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

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

 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 i)rimary 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. 



7, hind - gut ; 8, calcareous wheels ; 

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

 canal. 



