560 E. W. MACBRTDE. 



five-lobed, so that each somewhat resembles the whole organ 

 in an earlier stage. Muller was led astray by this resem- 

 blance and gave to each primary lobe the name "palmate 

 organ/' and he imagined that the palmate organ of the 

 earlier larva was identical with the first of these, and that the 

 rest were formed de novo. He described also a semi- 

 circular wavy fold of skin, from which the rudiments of the 

 adult arms grew oat, to which the " palmate organs " became 

 co-adapted later. These latter gave rise to the radial canals 

 and tentacles of the water-vascular system. 



The larva of Ophiothrix fragilis was recognised by 

 Muller owing to its possessing on the fi.rst tentacles of the 

 developing brittle-star the papillae characteristic of this species. 

 Muller only obtained late metamorphosing stages, and his 

 description concerns itself chiefly with the elaboration of the 

 details of the brittle-star. The last Ophiurid larva described by 

 Muller is one which he called the " worm-like larva," which he 

 imagined to belong to the Asteroidea (28). It Avas re-dis- 

 covered by Krohn (15), who proved that it was the larva of 

 an Ophiurid. In 1900 Caswell Grave (12) found a similar 

 larva on the American coast, and showed that it was the 

 young stage of Ophiura brevi spina. Owing to the 

 opacity of this larva neither Muller nor Krohn made out 

 much of its structure. 



The next great advance in our knowledge of Ophiurid de- 

 velopment was made by Metschnikoff in 1869 (21). In a 

 paper on the development of Echinoderras and Ncraertines 

 he records the results of investigations on all four classes of 

 Echinoderm larva), as well as on the embryos of the viviparous 

 species, Amphiura squamata. Three kinds of Ophiurid 

 plutei are described, two of which are identified with larva3 

 described by Johannes Miiller. Of only one form, however, 

 did he obtain suflEicient stages to describe the development in 

 any detail, and this is the Pi u tens bimaculatus of Miiller. 

 In the youngest stage which he obtained he was able to make 

 out the right and left coclomic cavities lying at the sides of 

 the larval oesophagus. In the next stage he discovered a 



