62 SUMMARY OF CURRENT RESEARCHES RELATING TO 



counted. He suggests that the abundant suspended particles in the 

 canal-water of Suakim caused the contractions which Keller regarded 

 as rhythmic. On experimental and histological grounds Krukenberg 

 regards their existence as very improbable. 



Porifera. 



Sponges.* — Prof. W. J. Sollas has a well-illustrated general article 

 on Sponges. In the account of structure and form he commences with 

 a description of Ascetta primordialis, as a simple sponge ; the various 

 modifications undergone by the canal-system are next described, in 

 connection with which the term of prosopyle is applied to the pores 

 which lead directly into the radial tubes or paragastric cavity. In the 

 skeleton, megascleres or skeletal, and microscleres or flesh, spicules are 

 distinguished ; the modifications of these are described, considerable 

 additions being made to the terminology of the skeletal constituents. 



In the account of the histology of the mesoderm various kinds of 

 cells are distinguished ; the stellate connective-tissue corpuscles aro 

 called collencytes, and the tissue collenchyme. Cystenchyme consists 

 of closely adjacent large oval cells, and is particularly found in certain 

 TYtractinellids. Long fusiform connective-tissue cells are called desma- 

 cytes ; they often form the greater part of the cortex of a sponge. In all 

 higher forms contractile fibre-cells or myocytes are to be found, and 

 there appears to be more than one kind of them. The supposed sense- 

 cells are called aesthacytes. 



With regard to protoplasmic continuity, Prof. Sollas says, " In most 

 sponges a direct connection can be traced by means of their branching 

 processes between the collencytes of the mesoderm and the cells of the 

 ectodermal and endodermal epithelium and the choanocytes of the flagel- 

 lated chambers. As the collencytes are also united among themselves, 

 they place the various constituents of the sponge in true protoplasmic 

 continuity. Hence we may with considerable probability regard the 

 collencytes as furnishing a means for the transmission of impulses ; in 

 other words, we may attribute to them a rudimentary nervous function." 



The extraordinary profusion of sponge-spicules in some modern 

 marine deposits and in the ancient stratified rocks is accounted for by 

 the fact that the sponge is constantly producing and disengaging spicules. 

 Each spicule originates in a single cell or scleroblast. 



The phylum Parazoa or Spongise is thus divided : — 

 Branch A. Megamastictora. Branch B. Micromastictora. 

 Class. Calcarea. Class I. Myxospongise. 



„ II. Silicispongiae. 

 Sub-class i. Hexactinellida. 

 „ ii. Deruospongiae. 

 Tribe a. Monaxonida. 

 „ b. Tetractinellida. 



A sufficiently detailed systematic classification is given. 



The asexual and sexual modes of reproduction are described, and a 

 notice is given of the two chief types of development ; one, which is 

 common among the calcareous sponges, is characterized by the " aniphi- 

 blastula," and the other by the " planula " stage. 



A short account is given of the little that is known as to the physiology 



* Eucycl. Brit., xxii. (1887) pp. 412-29. 



