SPICULES OF SYNAPTA AND AURIOULARIA LAHVA. 499 



in the pair of processes situated at the aiial^ i.e. lowei% 

 extremity of the larva. Both wheels and globes abut against 

 the ectoderm of the larva^ though they arise from cells 

 internally situated, i.e. from the mesenchyme cells. But 

 before describing the development of the spicules I will 

 quote the relevant passages of Chun's account of this subject, 

 which, though incorrect as regards the main point, yet contains 

 many true observations. " At the time of the appearance of 

 the first calcareous wheels the cellular elements of the gela- 

 tinous substance (filling the interior of the larva) are sharply 

 differentiated into skeletogenous and connective-tissue cells. 

 The latter possess several long processes, which are much 

 ramified, and are interwoven almost after the manner of felt; 

 the skeletogenous cells, on the contrary, are spherical and 

 surrounded by a distinct membrane, in consequence of which 

 they emit no pseudopodia. The sharp histological differentia- 

 tion of the mesoderm cells, which was certainly preceded by 

 an indifferent stage, may be essentially due to the fact that 

 the calcareous bodies originate at a remarkably late period in 

 comparison with what is found to be the case in other Echino- 

 derra larvae. The skeletogeuous cells accumulate . . . close 

 beneath the ectodermal epithelium,'' " A richly vacuolate 

 plasma at once distinguishes the skeletogenous cells, the 

 average size of which is 0"01 mm. 'l^hey rapidly grow to 

 twice and thrice this bulk, while simultaneously the number of 

 the cell-nuclei increases. In the same Auricularia we meet 

 with all intermediate stages between uni- and multinucleate 

 cells, which at first still retain a rounded contour, but subse- 

 quently flatten out on one side and become cup-shaped. The 

 nuclei measure from O'OOS mm. to 0-004 mm. in length, and 

 originally (so long as only from two to four are present) 

 occupy a peripheral position; they afterwards increase to 

 from six to eight in the case of the Mediterranean Auricularite, 

 and to from twelve to eighteen in that of those from the 

 Canary Islands, and form a central nuclear cluster. When 

 the cells have attained a size of 0'03 mm. there appears within 

 the old cell-membrane a new one, which has an undulating 



