502 



INVEETEBKATA 



CHAP. 



looks like a young Asteroid, and for this reason Johannes Miiller 

 included the " worm-like larva " amongst the Asteroid larvae. 



As development proceeds the anterior part of the larva diminishes 

 in size, and the adult arms grow out. The epineural spaces appear 

 at first as open grooves, the edges of which meet. The perihaemal 

 spaces arise exactly as in Ophiothrix. 



The only strikingly abnormal features about this development are 

 the method of the gastrulation and of the formation of the coelom. 

 With regard to the first point, it is a remarkably interesting fact that 

 if the eggs of Ophiothrix be removed from the ovary and artificially 



fertilized, instead of being 

 allowed to be spawned natur- 

 ally, they often exhibit the 

 same kind of early develop- 

 ment as Grave has described 

 for Opliiura brevispina. The 

 mesenchyme begins to be cut 

 off in the early stages of 

 segmentation, so that the 

 blastocoele, from its very 

 beginning, is filled with it, 

 and the blastula is in reality 

 a " morula." Invagination is 

 a solid inwandering of cells, 

 and, when this solid mass 

 becomes hollowed out, a 

 tongue of cells persists which 

 projects into the lumen of 

 the archenteron. Such larvae 

 never develop the anterior 

 vacuolated crest. 



This interesting fact leads 

 us to form some such con- 

 clusion as this. The processes 

 of growth and cytotaxis, which 

 are the underlying causes of 



the outward and visible "signs" of formation of mesenchyme, in- 

 vagination, etc., are the same over a wide range of species. The 

 relations of these processes to each other in degree of intensity 

 and times of maximal intensity, in a word the differential equations 

 which connect them, differ as we pass from species to species, and in 

 consequence the same processes give rise to very different visible 

 results in different cases. By interfering with the normal relations 

 in any one species, in the case of Ophiothrix, by fertilizing the egg 

 before it is quite ripe, the conditions usually found in quite a different 

 species may be reproduced. 



With regard to the formation of the coelom, we have learnt how 

 profoundly this process may be altered amongst Asteroidea by the 



FIG. 382. Ventral view of the larva of Ophiura 

 brecis during the metamorphosis. (After 

 Grave. ) 



as.?, rudiment of azygous tentacle; b.t, rudiments 

 of buccal tentacles ; cil, transverse ciliated bands ; 

 u, mouth. 



