482 



INVEKTEBEATA 



CHAP. 



ac. 



on 'the primitive condition of the development of Asteroidea. 

 Gemmill's researches (1912) on Solaster allow of another interpreta- 

 tion of the type of development. In Solaster the posterior coelorn 

 is not symmetrical, but inclined to the left, and the area which 

 eventually forms the gut does not extend evenly all round the 

 arbhenteron. We might therefore derive the condition of affairs in 

 the larva of Solaster from the condition of things in the larva of 



Asterina, by imagining that the 

 preponderant growth of the organs 

 of the left side has been pushed 

 so far back into the embryonic 

 period, that the gut-rudiment is 

 swung out of the longitudinal 

 into a transverse position, and so 

 the open end, from which the 

 coeloni is cut off, is directed to the 

 left instead of anteriorly. 



Solaster is further remarkable 

 for the fact that, although it is 

 a star-fish with many rays, the 

 left hydrocoele has at first only 

 five lobes which all develop simul- 

 taneously, and additional lobes 

 are developed much later ; so that 

 in one and the same specimen, 

 when the more dorsal lobes of the 

 hydrocoele have already developed 

 lateral tube feet, and the peri- 

 haemal spaces in connection 

 therewith have been completely 

 separated from the coeloni, the 

 more ventral lobes will still be 

 quite undivided, and the ad- 

 jacent perihaenial spaces will 

 have the form of shallow evagina- 

 tions of the coelom. This circum- 

 stance seems to indicate that the number five, so characteristic of 

 the rays in all Echinodermata, was characteristic also of the ancestral 

 Echinoderm, and that where many rays are found this is not a 

 survival of a primitive state of affairs, but is a secondary 

 modification. 



EXPERIMENTAL EMBRYOLOGY OF ASTEROIDEA 



A great many experiments have been performed on the eggs of 

 Asteroidea, but in the case of many of these quite the same results 

 have been obtained with the eggs of Echinoidea, which have been 

 classic subjects of experimentation since the study of Experimental 

 Zoology started. So far as space will allow they will be mentioned 



FIG. 372. Longitudinal frontal section of 

 the larva of Solaster endeca. (After 

 Gemmill.) 



a.c, anterior coelom ; al, alimentary canal ; 

 hy, hydrocoele ; l.p.c, left posterior coelom ; 

 r.p.c, right posterior coelom. 



