ECIIIXODERMS. 137 



and the creature having lost its apparatus for motion sinks to the 

 bottom : tentacles in the meanwhile, five on each side of the mid- 

 line, have been developed, and hooks are seen at the two extremities 

 of the body which shew by their peculiar form that those extremities 

 correspond to the ends of the arms : the embryo now cup-shaped 

 from the increased convexity of its dorsal surface attaches itself to 

 the bottom by this surface from which it secretes its pedicle. The 

 absence of symmetry in the relation of the Echinoderm to its larva 

 is indicated by the stern of the Echinoderm being placed at right 

 angles to the axis of the larva, and the tentacles and mouth on the 

 opposite surface 1 . 



In by far the greater number of Echinoderms the embryos pass 

 gradually into forms which, however remarkably they may differ, 

 are all laterally symmetrical. The axis becomes bent and on the 

 ventral surface (that where the mouth opens) is a depression bounded 

 above and below by transverse bands of cilia which are continuations 

 of the lateral bands which bound the dorsal surface. They all have 

 a complete digestive tube consisting of mouth, oesophagus, stomach, 

 intestine and anus. This tube is placed in the median plane, the 

 mouth in the ventral depression described above, and the tube 

 curves from it to terminate beyond the transverse band of cilia 

 above the mouth on the same ventral surface. Also they have all 

 an aquiferous system, a tube terminating externally in a dorsal pore 

 and internally in a sac. When MUELLER observed the singular 

 forms of the larvae of Ophiurce, and Echini with their long processes 

 supported by slender rods of carbonate of lime he named them 

 Pluteus from their general resemblance to a painter's easel with his 

 work upon it. In Asterice and Holothurice the larvae have a more 

 flattened form, like a coat of arms witli its surrounding ornaments. 

 The process of development in these different larval forms is two- 

 fold. In the first case the body of the Echinoderm is formed by 

 gemmation round the stomach of the larva, which continues to be 

 its stomach, and when it is formed, all that remains of the larva, 

 with the exception of certain structures in connexion with the aqui- 

 ferous system is gradually ( Ophiura and Echinus) or simultaneously 

 (Bipinnaria asteriyera} rejected. In the second case the symmetrical 



1 [Coinp. Beobacli. ubcr Anatomic u. Entwickeluny einiger ivirbcllosen Secthicrc von 

 DR. W. BUSCII. fol. Berlin, 1851, s. 8288.] 



