JIICROSCLERES. 



85 



d. Chela a move or less curved shaft, bearing at each end a variable number 

 of recurved processes. 



2. Asters are multiactinate microscleres. There are two chief kinds ; (A) 

 asters or euasters, in which the actines proceed from a centre, and (B) strepi.- 

 asters, in which the actines proceed from an axis which is usually spiral. 



A. Euasters are in many varieties ; but of these we need only note the 

 stcrrastcr, in which the actines are numerous and soldered together by sub- 

 sequently deposited silica, which extends almost as far as their extremities (29). 

 Other varieties are the chiaster (17), the pycnaster (36), the oxyaster (S3), and 

 the sphcraster (32). 



I \ 



FIG. V4. Development of Sycon raphanus (after P. E. Sch.). a, ripe ovum; 6, stage with 

 four segments ; c, stage with sixteen segments ; d, blastosphere with large dark granular 

 cells at the open pole; c, free-swimming larva, one half of the body (endodennal) being 

 formed of long ciliated cells, the other (ectodermal) of large granular cells. 



B. The Streptasters are also various. There are the spirastcr (12], the 

 metaster (14), the plesiaster (15, 16), the sanidaster (29), and the amphiaster (13). 

 In the amphiaster the actines form a whorl at each end of the axis, which is 

 straight. 



C. Reduced asters, in which the actines are few and variable. Thus we get 

 microrhabds, microcalthrops (26-31), microtriods (24). There are several varieties 

 of the microcalthrops depending on the branching of some of the actines. 

 Thus there is the monolophous microcalthrops (86) with one cladose (branched) 

 actine, the dilqphous (27), trilophous (30), and tctralophous (31) microcalthrops 

 with two, three, and four cladose actines. The candelabrum (35) is a tetralophous 



