124 



ZOOLOGY 



larval form known as the parenchy inula. The parenchymula 

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

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

 two posterior granular cells. In this condition it becomes fixed, 



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 part 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 

 there are also amoeboid cells derived from the two original granular 

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

 scleroblasts are formed of certain ectoderm cells which migrate 

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

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

 becomes completed by the enlargement of the internal cavity 

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

 the development of the osculum. 



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

 those in Glathrina hlanca, and the embryo leaves the parent sponge 

 in the peculiar stage to which the name of amphiblastula is 

 applied. When the blastula is formed, the greater part of its wall 

 consists of clear cells, with a number of granular cells — the archoeo- 

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

 and flagellate. The archoeocytes pass into the internal (segmenta- 

 tion) cavity and become completely enclosed by the flagellate 

 cells (stage of so-called pseudogastrula). 



The cells at the posterior end then lose their flagella and 

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

 wall of the embryo comes to be composed in one half of the 

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

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



2><?c 



Fig. 93. — Median longitudinal 

 section of the parenehyinula 

 larvaof Clathrinablanca. 

 p.g.c, posterior granular cells 

 which give rise to the archseo- 

 cytes. (From the Cambridge 

 Natural History, after Min- 

 chin.) 



