506 COMPARATIVE ANATOMY chap. 



parts is purel}' voluntary (i.e. results from causes entirely within the animal itself) 

 and not to some extent due to external stimuli, however slight. In any case, the 

 final result of the regeneration which follows is the same — the multiplication of 

 individuals. 



Fission of the body into two halves of approximately equal size with subsequent 

 regeneration has been observed in OpJiiuroidea, Asteroidea, and Bolothurioidea . 

 In the two former classes the plane of fission passes through the middle of the disc 

 (through the mouth and digestive sac), in the Bolotlmirioidea (Cucuinaria) it jjasses 

 transversely through the tubular body, dividing it into an anterior (oral) and a 

 posterior (apical) half. 



In the OpJmiroidea, reproduction by means of fission has been observed in the 

 following genera : Ophiactis {Millleri, Savigny, virciis), OpMociiida (sexradia), Ophio- 

 coma {pmnila, Valencice), Ophiothela [isidicola, dividica). 



Among the Asteroidea, schizogony is specially characteristic of many species of the 

 genns Astei'ias (acutispiua, atlantica, calamaria, microdiscus, tenuispina), and is also 

 found in Asterina IVcga, Cribrella sexradiata, Stichaster albulus. 



Another kind of asexual reproduction seems to be very common in the family of 

 the Linel-iidce. In these Asteroids, the arms become constricted off at their bases, 

 after which not only does the disc regenerate the arms which have been cast off, but 

 each individual arm regenerates the disc and the other arms ("comet" forms of 

 Asteroids, Fig. 397 A, B). 



Asexiial reproduction does not, as a rule, appear to take place simultaneously with 

 sexual reproduction, but there are exceptions to this rale. 



XXI. Ontogeny. 



In all Echinoderms, except those few forms in which care of the brood occurs, the 

 fertilised eggs develop into free-swimming, bilaterally symmetrical larvae, which are 

 transformed into the radially built Echiuoderm after passing through an often very 

 complicated metamorphosis. 



The larvje of the different classes of Echinoderms will first be compared ex- 

 clusively according to their external characteristics. 



A. The various Larval Forms of the Eohinodermata. 



We shall first construct a hypothetical larval form, and then deduce the various 

 larval forms from it (Fig. 398, A). 



The body of the larva is egg-shaped, and concave on the ventral side. In the 

 base of the concavity lies the larval mouth. Near one of the poles {i.e. near the 

 posterior end), but still on the ventral side, there is a second aperture (proceeding 

 from the blastopore of the gastrula larva) ; this is the larval anus. A ciliated band 

 which runs back upon itself surrounds the mouth along the edge of the ventral con- 

 cavity ; posteriorly it runs over the ventral side in front of the anus, and is the 

 circumoral ciliated ring. The aperture of the mouth and its immediate surround- 

 ing are also ciliated (adoral ciliated band). 



1. Bolothurioidea. — The Holothurid larva known as Auricularia (Fig. 398, A) 

 differs but little from the hypothetical form. The ventral concavity becomes more 

 complicated, lengthening on each side posteriorly and anteriorly, while a posterior 

 median portion with the anus forms a ventral prominence, the anal area, and a 

 median portion in front of the mouth forms another prominence, the preoral area. 

 The ciliated band which runs longitudinally along the ventral depression assumes 



