THE STAEFISH SOL ASTER ENDECA. 41 



interest and importance is the constant position of the anus alike in S. endeca and 

 S. papjjosa (p. 23). 



VII. Development of the Skeleton. (PI. I. figs. 8, 9, 12 ; PI. II, figs. 19 & 20.) 



Representatives of three sets of skeletal elements, namely the ambulacral plates, 

 the plates of the dermal reticulum, and the spines, make their appearance indepen 

 dently two or three days before the end of the free-swimming stage. 



Ambulacral Plates. — The ambulacral plates develop in the mesoderm of the left 

 side of the larval body, where they form a crescent similar in curvature to the 

 hydroccele, and, like the hydrocoele, having the gap between its horns opposite to 

 the neck of the larva. As in other Starfish, the innermost or proximal elements 

 in each ray are first laid down, the sequence in Solaster corresponding with that in 

 which the radial pouches themselves are formed. Five pairs thus appear almost 

 simultaneously in connection with pouches I to V of the hydrocoele, there being, 

 however, delay in the formation of the first plate of the first pair in the series, 

 i. e., that adjacent to the larval neck. A sixth pair appears at the time of fixation, 

 but the ninth is only added after the mouth has formed. The first ambulacrals are, 

 of course, to the oral side of the first tube-feet. 



A second pair of ambulacrals appears in each ray shortly after the ray has acquired 

 its second pair of sucker-feet, and so on with succeeding pairs, the formation of the 

 ossicles being later than that of the correspondingly numbered feet. Thus in a 

 specimen four months old there were three pairs of ambulacrals, and at seven months 

 the number had increased to five. The bodies of the first and second ossicles in each 

 row have united together by the fourth month. 



The first pair of adambulacrals appears in each interradius at much the same time 

 as the second ambulacrals in the adjacent rays. Succeeding adambulacrals appear in 

 similar sequence. The first pairs are much larger than the others, and they form the 

 teeth. Small groups of spines early become associated with each of the adambulacrals. 

 Those associated with the first pair are longer and stronger than the others, and two 

 or more of these in each interradius project horizontally inwards towards the centre of 

 the mouth. These form what may be called the denticles, and their development is 

 slightly different from that of ordinary spines (p. 43). 



Dermal Reticular Skeleton. — The plates of the dermal reticular skeleton develop in 

 the deeper mesodermic layer of the aboral body-wall. Towards the end of the free- 

 s-wimming stage on the right side of the larva one finds a middle area with many such 

 plates in process of formation. This area overlies the right lateral diverticulum of the 

 anterior coelom (future epigastric coelom). It is separated by a narrow plateless 

 interval from a marginal area overlying the posterior coelom (future hypogastric 

 ccelom), in which five or six pairs of small plates may be made out. Comparison with 

 later stages shows that these pairs are destined to be carried outwards at the tips of 



VOL. XX. — PART I. No. 6. — February, 1912. G 



