464 INVERTEBEATA chap. 



Ophiuroidea, Echinoidea, and Holothuroidea. It consists of two 

 sides, and of an anterior and of a posterior cross-bar. The anterior 

 cross-bar is situated in front of the mouth, and the posterior cross- 

 bar in front of the anus. Neither cross-bar is straight, both are 

 bent into the form of loops. The loop formed by the anterior cross- 

 bar is bent back along the ventral surface of the prae-oral region or 

 forehead of the larva, and it is termed the prae-oral loop, and the 

 area which it surrounds is termed the frontal field. The loop 

 formed by the posterior cross-bar bends forward along the ventral 

 surface in front of the anus, and the area which it surrounds is called 

 the anal field. 



When the alimentary canal has been completed by the union of 

 the stomodaeum and oesophagus the larva is able to feed, and, if 

 suitable diatoms be provided, it will live and grow, even in a com- 

 paratively small aquarium, without any change of water. Dr. 

 G-emmill has reared the larvae of Asterias ruhens for over two months 

 in his laboratory at Glasgow, until they had completed their meta- 

 morphoses ; and this feat has also been accomplished with the larvae 

 of Asterias glacialis by Professor Yves Delage in his laboratory at 

 Eoscoff. 



As the larva increases in size the tissue of the longitudinal ciliated 

 band grows more quickly than adjacent regions of the ectoderm, and 

 the band becomes thrown into folds. These folds form lobe-like out- 

 growths termed larval arms, which correspond in number, position, 

 and size on the two sides of the .body, and confer on the larva a 

 " bipinnate " appearance, whence the name Bipinnaria. Mortensen 

 (1898) has invented a nomenclature for the larval arms of Asteroidea 

 and the homologous structures in other Echinoderm larvae, and we 

 shall follow his nomenclature in this book. 



Before the larval arms have attained any size the prae-oral loop 

 becomes separated from the rest of the longitudinal ciliated band, and 

 this primary band becomes, in this way, divided into two secondary 

 bands, which we shall term the prae-oral and the post-oral bands 

 respectively (pr.o.h, p.o.b, Fig. 358). 



In the Bipinnaria larva of Asterias vulgaris the prae-oral band 

 carries only two larval arms, which are termed the prae-oral arms 

 (pr.o.a, Figs. 358, 359), and the frontal field is somewhat quadrangular 

 in shape. But in the larvae of the European species, Asterias ruhens 

 and A. glacialis, the prae-oral band develops in addition a median 

 anterior arm, directed forwards, termed the median ventral arm, 

 and the frontal field is consequently triangular in shape. From the 

 post-oral band, in all three species of Asterias, there is developed an 

 anteriorly directed arm, which is termed the median dorsal arm 

 {m.d.a. Fig. 358). From the sides of the post-oral band, about one- 

 third the length of the larva from its anterior end, there are given 

 off two arms, termed the antero-dorsal arms {a.d.a, Fig. 358). Still 

 farther back, at rather more than two-thirds the length of the larva 

 from its anterior end, two siroila.r ftrm§ arise termed the postero- 



