THE 8TAEFISH SOLASTBE ENDECA. 43 



to lie along the strands or at less nodal points of the meshwork. In cleared tissues the 

 former stand out as being slightly larger, more angular, and more densely calcified 

 than the rest, as well as associated with larger rosettes of spines. They occur all over 

 the surface, as well as roughly in radial and interradial rows. Sometimes also they 

 form a ring about halfway between the centre and the margin of the disc. Possibly 

 this ring has some relation to the former boundary between the right lateral and the 

 posterior coeloms. 



To the dermal skeletal system of plates belong also the plates with which the 

 interbrachial septa are strengthened. The lowest of these lies between the first two 

 adambulacrals forming the so-called odontophore. 



The Spines. — The spines appear as small nodules in the superficial mesodermic 

 layer of the aboral body-wall. At first they are uniformly distributed, and twice or 

 thrice as numerous as the dermal plates of the deeper layer (PI. I. fig. 8). By the 

 time of fixation their position is marked by small papillee standing out everywhere 

 from the aboral surface. 



Later, the papillee increase in length (PL I. figs. 12, 14), and the jagged tips of the 

 spines stick out from their summits. New papillae also appear in the body-wall 

 wherever space allows. In the fourth and succeeding months the spines form a very 

 prominent feature in the external appearance of the young Starfish (p. 24, PL I. 

 fig. 14). 



Meantime, the inner ends of the spines tend to cluster together in twos and threes, 

 and each cluster becomes associated with one of the deeper plates, the latter developing 

 a rounded knob for its support. During later growth new clusters appear and new 

 spines are added to existing clusters. In the adult the clusters over the main nodal 

 plates contain from 7 to 9 spines, while over the rest the number is less or the clusters 

 are absent altogether. 



The mode of development of the plates and of the spines closely resembles that 

 described in other Starfish, e. g., by Ludwig in Asterina (12), and need not be repeated 

 here. It may be noted, however, that those which project horizontally oralwards 

 from the first adambulacrals are flattened slightly in the horizontal plane, and in their 

 first appearance resemble an ordinary dermal plate rather than the little closed star or 

 cartwheel which forms the base of the typical spine. 



VIII. Histology. 



A number of points may perhaps be npiost conveniently grouped together under the 

 heading of Histology. These are : the mesenchyme and the disposal of the yolk; the 

 statolith C?) ; the central nervous system ; the larval muscular system ; ciliation of 

 internal cavities. 



Mesenchyme and Yolk. — It will be remembered that during segmentation the whole 

 of the ovum is divided up into cells, and that the cell-contents at the upper and lower 



g2 



