XIV] LARVAL FORMS 365 



down and developing into little pentacrinoids with a jointed 

 stalk. The name "pentacrinoid" is suggested by their resem- 

 blance to Pentacrinus. These stalked young present interesting 

 features in the skeleton found in many living and fossil Crinoidea 

 but absent in the adult Antedon. Thus the mouth is guarded by 

 five interradial valves each supported by an oral plate, and the 

 rosette is represented by five ossicles. 



Leaving aside the Crinoidea, the development of which is known 

 only in one case and is there evidently much modified, the eggs of 

 the other four groups of Echinodenns develop into free-swimming 

 animals which for periods varying from a fortnight to six weeks lead 

 a free life at the surface of the ocean. These young are called 

 Dipleurula larvae and they are, as mentioned, utterly different from 

 adult Echinoderms: unlike these, but like most other animals, they 

 are bilaterally symmetrical (Fig. 172). They swim by means of a 

 powerful longitudinal ciliated ring, drawn out into a number of 

 arms or processes. They possess a complete alimentary canal, 

 consisting of oesophagus, stomach and rectum, while the coelom is 

 represented by two sacs lying one at each side of the digestive 

 tube. These sacs, as a study of the early development teaches, 

 are portions of the alimentary canal budded off from the rest. One 

 of the most interesting features in the development is the fact that 

 these sacs undergo transverse division in the 'same way as do the 

 mesodermal bands of an Annelid. On the left side three segments 

 are formed, on the right side usually two, but in the Dipleurula larva 

 of Ophiuroids three, of which the middle one has usually a transient 

 existence. The most anterior on each side often coalesce to form 

 a median sac into which the originally single madreporic pore opens 

 on the left side; in the Dipleurula larva of Asterias a similar pore 

 is formed on the right side, but this has only a transient existence ; 

 a portion of this sac becomes the axial sinus of the adult. The 

 middle section on the left side takes on the form of a curved wreath 

 with five lobes projecting from it ; by the union of the two ends of this 

 wreath it is converted into a ring which becomes the future water- 

 vascular ring, whilst the lobes form the radial canals of the adult. 

 This wreath is termed the hydrocoele. The transient middle section 

 of the coelom, on the right side in the larvae of Ophiuroidea, some- 

 times forms a similar hydroeoele, which is then termed the right 

 hydrocoele, and occasionally a middle section of the coelom is cut 

 off on the right side in the larvae of Asteroidea and Echinoidea 



