2i 



ZOOLOGY 



irval form known as the pnrenchyimda. The parenchymula 

 Fig. 93) consists of three kinds of cells : — (1) an external layer 

 f flagellate cells ; (2) an inner mass of amoeboid cells ; (3) the 

 wo posterior granular cells. In this condition it becomes flxed, 

 and develops into the form of a flat plate 

 with an irregular outline. Most of the 

 amoeboid cells now migrate to the outer 

 surface, passing between the flagellate 

 cells and then becoming arranged outside 

 them to form the ectoderm. The flagel- 

 late cells now form an irregular mass 

 together with a number of non-flagellate 

 cells derived from the ectoderm, which 

 are destined to give rise to the porocytes. 

 A cavity appears in the mass, and becomes 

 surrounded by a layer of porocytes. The 

 cavity increases in size, and is soon seen 

 to be bounded not by the porocytes alone, 

 but in 23art also by flagellate cells. Sub- 

 sequently the flagellate cells come to 

 form the entire boundary of the cavity, 

 the porocytes passing outwards to become 

 perforated by apertures — the inhalant 

 apertures — in the wall of the sponge. 

 Among the flagellate cells and porocytes 

 here are also amoeboid cells derived from the two original granular 

 ells; some of these give rise to the reproductive cells. The 

 cleroblasts are formed of certain ectoderm cells which migrate 

 awards, and at an early stage arrange themselves in threes to give 

 ise to the tri-radiate spicules. The development of the sponge 

 lecomes completed by the enlargement of the internal cavity 

 paragastric cavity) which is now lined by flagellate cells, and by 

 he development of the osculum. 



In Sycon the early stages (Fig. 94, a-e) differ somewhat from 

 hose in Clathrina hianca, and the embrj'o leaves the parent sponge 

 a the peculiar stage to which the name of (imphiblastula is 

 pplied. When the blastula is formed the greater part of its wall 

 onsists of clear cells, with a number of granular cells — the archfeo- 

 ytes — at the posterior pole. The clear cells become elongated 

 nd flagellate. The archaeocytes jsass into the internal (segmenta- 

 ion) cavity and become completely enclosed by the flagellate 

 ells (stage oi s,o-caX\ed pseudogastrula). 



The cells at the posterior end then lose their flagella and 

 lecome large rounded granular cells, so that after a time the 

 /all of the embryo comes to be composed in one half of the 

 lagellate cells that have remained unaltered, and in the other half of 

 he large granular cells. It is in this stage — termed the amphi- 



;p.r/.C 



[G. 93. — Median longitudinal 

 scctinn uf the parenchyiriuLi 

 larva of Clathrina blanca. 

 p.g.c.^ pusteriox" granular cells 

 which give rise to the urch?eo- 

 cytes. (From the Cambridge 

 Natural Histoi-y, after Min- 

 chin.) 



