490 INVERTEBKATA CHAP. 



wrongly. A year later, however (1846), he bestowed the same name 

 on the similar larva of an Echinoid, which, though generally homo- 

 logous with the larva of an Ophiuroid, differs from it in most 

 important points of detail. The confusion of having the term 

 " Pluteus " applied indifferently to both forms of larvae, led Mortensen 

 (1898) to propose that the single term Pluteus should be replaced 

 by the terms Ophiopluteus and Echinopluteus, as applied to 

 Ophiuroid and Echinoid larvae respectively, and this excellent 

 practice will be followed here. 



When the larva has become endowed with a complete alimentary 

 canal it is able to feed, and it grows rapidly in size. The four arms 

 increase in length, but the first-formed pair, the postero-lateral, grow 

 very much more quickly than the others. The coelouiic sacs grow 

 in length and extend backwards along the sides of the oesophagus, and 

 beyond the junction of this with the stomach to the sides of the latter. 



When the larva is about a week old important changes occur. 

 Two additional pairs of arms are developed ; of these one pair are 

 situated at the sides of the oesophageal region ; they correspond 

 roughly in position to the appendages of the Bipinnaria known as 

 postero dorsal, and receive the same name. The other pair arise in 

 front of the anus, from the posterior cross-bar of the ciliated band, 

 and are termed post-oral, since they correspond to the similarly 

 named appendages of the Bipinnaria. Each new arm is supported 

 by a calcareous rod which is developed as a new branch from one of 

 the two original calcareous stars. The rods supporting the post-oral 

 arms take their origin from the centre of the star, on each side, which 

 thus becomes quadri-radiate, but those supporting the postero-dorsal 

 rods spring, as branches, from the rods which support the antero- 

 lateral arms. 



Whilst these changes have been occurring the coelom has been 

 undergoing further development. First on the left side and then on 

 the right, the coelomic sac becomes transversely segmented into 

 anterior and posterior divisions. The posterior divisions apply 

 themselves to the sides of the larval stomach, which has now become 

 globular ; thus these posterior divisions are somewhat widely separated 

 from the anterior divisions, which lie close to the larval oesophagus, 

 just over the region occupied by the posterior part of the ad oral 

 ciliated band. 



It is obvious that the anterior part of each coelomic sac corre- 

 sponds to the anterior coelom of the Bipinnaria, and the posterior por- 

 tion to the posterior coelom of that larva. From the inner walls of 

 the anterior coelom on both sides are developed muscular fibres which 

 form a series of circular muscles surrounding the oesophagus, and by 

 which its peristaltic movements are carried out. All the arms grow 

 rapidly in length, but the postero-lateral outgrow all the rest and the 

 V form of the larva is maintained. 



From the posterior ends of the right and left anterior coeloms a 

 thick-walled vesicle is nipped off. These are the rudiments of the 



