ECHINODERMA GENERAL DESCRIPTION 



5 



for the latter character, the Dipleurula agrees in essentials with 

 the larva of Enteropneusta, which was described by Job. Miiller 

 (1850) as an Echinoderm larva, under the name Tornaria 

 (Fig. II. 4). 



The simplest larval form among recent Echinoderms, that of 

 the Holothurians, known as Auricularia (Fig. II. 2, and Fig. III.), 

 differs from the Dipleurula in being bent upon its ventral surface, 

 so that the mouth lies in the middle of the concavity so formed, 

 while in front of it is a " preoral lobe," and behind it is a similar 

 prominence, in the middle of which, on the ventral surface, is the 

 anus ; the cilia are restricted to a band immediately surrounding 

 the mouth, and a band that 

 passes in front of the mouth, 

 then round the edge of the 

 ventral concavity, and across 

 in front of the anus. In 

 subsequent development the 

 ciliated ring becomes very 

 sinuous, and when the Auri- 

 cularia assumes a barrel 

 shape, before changing into 

 the Holothurian, the ring 

 atrophies in sixteen places, 

 and the separate pieces unite 

 in such a way as to form 

 five rings like hoops round 

 the barrel. In this stage the 

 mouth has again passed up 

 to the anterior pole, and the 

 anus down to the posterior. 

 This form is called the Pupa 

 (Fig. I. 8, p. 219). 



The only free larval form 

 that is known among Pel- 

 matozoa is that of the highly 

 specialised free-moving Crinoid, Antedon. It resembles the Holo- 

 thurian Pupa in general shape (Fig. IV.), and in the possession of 

 five ciliated bands (cc), probably derived in a similar manner ; 

 but since the early stages have been pressed out of the develop- 

 ment, this cannot be considered proven. Here, moreover, there 

 remains a ventral concavity, through which the definitive mouth 

 breaks (0) ; there is no anus at this stage. Anteriorly is a tuft 

 of long cilia. 



Most Asteroidea have a larva known as Bipinnaria (Fig. II. 

 3, arid Fig. VI. 10), which passes through an Auricularia stage. 

 By a meeting of the sinuosities of the ciliated ring anteriorly, 



FHJ. III. 



Aui-ifiiliti-in of Sijiiapta, the ventral surface and 

 left side facing the observer (after Semon). 0, 

 mouth ; oe, oesophagus ; ,<?, stomach ; /, rectum ; 

 lip, hydropore; he, hydrocoel ; r and Ipc, right 

 and left posterior coeloin ; n, nervous band; sp, 

 spicule in form of wheel ; c, ciliated bauds, viz. 

 1, preoral; 2, adoral ; 3, circumoral ; and 4, 

 anal. x25. 



