i86 



PORTFERA 



dermal layer includes a complete outer covering- of pinacocytes, 

 which is reflected over the oscular rim to meet the gastral layer 

 at the distal end of the tube ; a deeper gelatinous stratum in 

 which lie scleroblasts and their secreted products — calcareous 

 spicules ; and finally porocytes.^ These last are cells which traverse 

 the whole thickness of the thin body-wall, and are perforated by 

 a duct or pore. The porocytes are contractile, and so the pores 

 may be opened or closed ; they are a type of cell whicli is 



:nown oni 



Calcarea. 



}. 77. — The two types of Asconid 

 collar cells. A, of Clathrina, 

 nucleus basal ; B, of Leuco- 

 solenia, nucleus not basal, 

 liagellum arising from the basal 

 nuclear membrane. (A. after 

 Blinchin ; B, after Bidder.) 



It will be noticed that the fusiform or 

 stellate " connective tissue cells " are 

 absent. The layer of pinacocytes as 

 a whole is highly contractile, and is 

 capable of diminishing the size of 

 the sponge to such an extent as quite 

 to obliterate temporarily the gastral 

 cavity." 



The choanocytes show certain con- 

 stant differences in structure in the 

 families Clathriuidae and Leucoso- 

 leniidae respectively. In the former, 

 the nucleus of the choanocyte is 

 in the latter, it is apical, and 

 the flagellum can lie traced down to 

 it (Fig. 77). 

 The tetraxon spicules have " equiangular " triradiate systems in 

 the Clathrinidae, while in Leucosoleniidae they are "alate." Finally, 

 the larva of Clathrinidae is a "parenchymula" (see p. 226), that of 

 Leucosoleniidae an " amphiblastula." 



The fact that it is possible to classify the Calcarea Homocoela 

 largely by means of histological characters is in accordance with 

 the importance of the individual cell as opposed to the cell-layers 

 generally throughout the Porifera, and is interesting in serving 

 to emphasise the low grade of organisation of the Phylum. The 

 organs of sponges are often unicellular (pores), or the products of 

 the activity of a single cell (many skeletal elements) ; and even 

 in the gastral layer, which approaches nearly to an epithelium, 

 comparable with the epithelia of Metazoa, the component cells 



' Bidder, Quart. Journ. Micr. Sci. xxxii. 1891, p. 631, and Minchin, Quart. 

 Journ. Mia: Sci. xxxiii. 1892, jx 266. 



^ Minchin, Lankester's Treatise on Zoolorjy, p. 30. 



