64 SYNOPTIC COLLECTION. 



cells, and mesenteric pouches. This would place sponges 

 after the Hydrozoa and Anthozoa in a natural classifica- 

 tion, but the views of Marshall have not been established. 1 

 Biitschli and Sollas maintain that sponges belong to 

 an independent phylum, and give it the name of Parazoa. 2 



Owing to certain marked structural characters we have 

 considered the group as belonging to the Metazoa, but 

 as an independent and primitive group of this phylum 

 having more or less remote ancestral forms among the 

 Protozoa. At the same time it must be borne in mind that 

 the primitive characters are most plainly seen before the 

 sponge becomes a sessile animal, and that after fixation 

 takes place, certain adaptive characteristics and evidences 

 of reduction appear. It would seem as if the Porifera 

 and Coelentera, as descendants, speaking broadly, of the 

 Protozoa and Mesozoa, traveled along similar roads for a 

 short distance till the sedentary habits of the former and 

 the free-swimming, active life of most of the latter caused 

 a divergence of the roads. 



The processes by which the Metazoa have arisen from 

 the Protozoa through the Mesozoa have not been deter- 

 mined with certainty. The two leading views in regard 

 to the subject are those of Haeckel and Metschnikoff. 

 According to the gastraea theory of Haeckel the fertilized 

 egg of a Metazoan, a sponge for example, arises from an 

 unnucleated mass of protoplasm comparable with the 

 Monera of the Protozoa. Becoming nucleated and fer- 

 tilized, it may be compared with the adults of the most 

 specialized Protozoa. This egg becomes segmented, 

 thereby forming many similar but still united cells. These 

 resemble remotely a mulberry, so that the egg at this 

 stage is known as the morula. The cells arrange them- 

 selves about a central cavity filled with fluid, and this 

 stage is the blastula. Next the cells 'at one pole of the 



1 See "The Relationships of the Porifera," Vosmaer, Ann. and 

 Mag. Nat. Hist., (5), XIX, 1887, p. 249. 



2 Reasons for this classification are given in the Rep. Chall. Exp., 

 Zool., XXV, 1888, p. xcii. 



