94 MARGAEET C. COLLCUTT. 



The muscle tails iu iiou-macerated material are most easily 

 demonstrable in sections of dactylozooids, where they are 

 greatly developed (PI. 1, fig. 8). 



c. Histology of the Mesogloea. 

 The mesogloea extends between ectoderm and endoderm 

 throughout the colony as a structureless, homogeneous layer. 

 In the basal part of the colony it is very thin, and, as men- 

 tioned above, thin strands of its substance serve to fix the 

 ccenosarc to the chitinous skeleton ; in the polyps it forms 

 a much thicker layer (PI. 1, fig. 1, c). 



D. Histology of the Endoderm. 



1. Endoderm of the Basal Coenosarc. — The endoderm 

 lining the cavity of the ramifying tubes consists of a single 

 layer of cubical cells, with large central oval nuclei. The proto- 

 plasm is coarsely granular and abundant, and there are many 

 food granules. Other deeply staining gland-like cells occur 

 at intervals throughout the endoderm of these tubes (PI. 1, 

 fig. 1, b) ; these are probably similar to certain so-called gland- 

 cells which occur in the endoderm of the Gasterozooids. 



2. Endoderm of the Gasterozooids. — This endodermal 

 layer consists of long narrow cells, closely pressed against one 

 another, and somewhat irregular in shape ; they vary in length, 

 so that their free ends are not all on the same level. They 

 are greatly vacuolated, especially towards their free ends, 

 where they widen slightly. The nucleus is situated in the 

 middle region, and close to the border of the cell ; it is oval, 

 and has a granular reticulum, with one or more nucleoli. 



In the hypostome the endoderm is thrown into longitudinal 

 ridges, which project into its cavity; there are usually from 

 four to eight of these ridges, which then pass downwards and 

 lose themselves in the endoderm of the stomach. A transverse 

 section of the hypostome consequently represents the lumen 

 of the gut as being star-shaped. AUman (14) mentions that 

 similar endodermal ridges occur in the endoderm of other 

 gymnoblastic hydroids. 



