No. 3-] DEVELOP.UEXT OF MAR/A'E SPOXGES. 301 



those I watched did not develop any further than the stage 

 shown in Fig. 37. 



In his memoir on Spongilla, Gotte (6) claimed that the entire 

 ectoderm of the larva was lost, the inner mass of cells giving 

 rise to all the layers of the adult. This account was opposed 

 to the earlier one of Ganin (7) who described the larval ecto- 

 derm as retained and becoming the ectoderm of the adult. In 

 his preliminary paper on the development of Spongilla, Maas 

 (15) stated that the larval ectoderm was not thrown off, but 

 after loss of cilia and gradual flattening became the thin mem- 

 brane-like ectoderm of the adult ; and the excellent series of 

 figures given in his later paper (14) retrace the process step by 

 step. Some of the older writers, Metschnikoff (11) and 

 Schmidt (22) described a partial or complete loss of the larval 

 ectoderm in several silicious sponges during the metamorphosis; 

 Barrois (i) believed that in his Desmacidon and Isodyctia 

 larvae, the ectoderm was partially lost ; and among the more 

 recent investigators, Marshall (18) describes a partial loss of 

 the ectoderm in Reniera filigrana. On the other hand, the 

 flattening of the larval ectoderm and its transformation into 

 the adult covering, has been observed not only in the case of 

 Spongilla, but in other carefully studied silicious sponges : in 

 Chalinula, Keller (10), and Myxilla,Vosmaer(34). For the views 

 of Yves Delage and Maas on the relation of the larval ecto- 

 derm to that of the adult in Esperia, reference may be made to 

 pp. 317-319. It seems to me that the alleged cases of total or 

 partial loss of the larval ectoderm (ectodermic hernia) so com- 

 pletely lack the requisite detailed proof, that none of them can 

 be accepted. In all such cases it is probable that the ectoderm 

 is not lost, but is flattened into an extremely thin membrane. 



Eclodenn. — In the flat epithelium into which the columnar 

 ectoderm changes, the separate cells are at first easily made out 

 (see PI. XVII, Fig. 36, longitudinal section of a larva like that 

 shown in Fig. 28). When the metamorphosis is complete, 

 however (see PI. XVII, Fig. 38, entire vertical section through 

 recently attached sponge, and PL XVII, Fig. 44, ditto through 

 an older sponge), the ectoderm on both upper and lower surfaces 

 forms a very thin membrane, in which nuclei are discernible 



