of the Ophiuridse and the Echinidse. 435 



a linear form, which returns into itself, ascends and descends 

 upon the arms in loops and passes from one arm to the other. 

 In the situation of the mouth it passes beneath it. Ciliary mo- 

 tion alone effects the entire locomotion of the animal ; all other 

 spontaneous motion is confined to the closure of the mouth and 

 oesophagus which takes place from time to time. The walls of 

 the stomach are of a granular or cellular structure and appear 

 green, the larva is otherwise perfectly transparent ; its azygous 

 apex and the extremities of all the arms are of an orange colour. 



Distinct indications of the nervous system were also perceived. 

 They consist of two small knots below the mouth, a right and 

 left, which are connected by a filament, and send several filaments 

 upwards towards the mouth and one downwards. 



These Ophiura larva? are not luminous. 



The first indication, both internally and externally, of the trans- 

 formation of the Plutevs into an Echinoderm, consists in the ap- 

 pearance of certain csecal figures with double contours at the sides 

 of the stomach and oesophagus. They are seen to form a row, first 

 upon one and soon afterwards upon the other side of the stomach 

 and oesophagus. The minute caeca are situated towards the ex- 

 terior ; their bases, which are connected together, are turned to- 

 wards the stomach ; each row appears like a thick membrane which 

 has been thrown into csecal folds. They soon completely sur- 

 round the stomach like a wreath. At first they do not project 

 beyond the surface of the Pluteus, but lie within its substance, 

 their contours being lost in it, but by their growth they soon 

 project beyond the surface of the Pluteus ; subsequently others 

 are developed which project beyond the crown formed by the 

 former ; these are neither more nor less than ten, each pair being 

 situated near each other : this is the first appearance of the arms. 

 The two belonging to each arm then fuse together, and the whole 

 assumes the form of a disc, which is grown over by five short 

 appendages. The former arms or columns of the Pluteus take 

 no part in this formation. The Pluteus holds the same relations 

 to the star-fish which is forming within it, as the embroidery 

 frame does to the embroidery which is worked within it. More- 

 over the arms of the Pluteus have no relation to the arms of the 

 Echinoderm. The latter is situated obliquely within the body of the 

 Pluteus, so that one of the arms of the star-fish crosses the great 

 axis of the Pluteus, and comes into view on the side of the azy- 

 gous apex of the Pluteus. As soon as the caeca become arranged 

 in the form of a crown or star, the deposition of lime occurs in 

 the form of ramified figures in the new formation ; during their 

 further development these assume the form of a lattice-work, as 

 is peculiar to the skeleton of Echinodermata. With the deve- 

 lopment of the minute caeca into a crown, distortion occurs at 



31* 



