22 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 



DESCRIPTION OF PLATES. 



PLATE II. 



Fig. 1. Ceratella fusca. Life size. The specimen of which this is a drawing 

 was washed up on Flinders Island, Bass Straits. The main stem of the colony 

 springs from a root-like structure made up of intertwined branches. On the left side 

 arises from the roots a very small independent stem. 



Fig. 2. Transverse section through a portion of the wall of an expanded 

 gastrozooid. Externally is the layer of ectoderm consisting of ciibical cells. The 

 mesoglcea is an extremely thin layer and the endoderm consists of large vacuolate 

 cells with granular protoplasm aggregated at their inner ends which face into the 

 gastral cavity. The nuclei are conspicuous and placed at the same end. Camera. 

 Zeiss E, oc. 2. 



Ed. ectoderm. End. endoderm. M. mesogloea. 



Fig. 3. A small portion of the external surface of a branch in the region in 

 which gonophores are numerous to show the external ectoderm containing thread 

 cells and a portion of one of the ccenosarc tubes. In the latter the endoderm forms 

 a definite layer of darkly-stained, small cubical cells. Surrounding this is the 

 ectoderm in contact with the external layer and containing developing thread cells. 



E. external layer of ectoderm. Eel. e'ctoderm of coenosarc tube. End. 

 endoderm. A. developing thread cells. Sk. skeleton. Camera. Zeiss E, oc. 2. 



Fig. 3a. Developing thread cells found in the ectoderm within the branches, 

 a. an ectoderm cell in which the micleus is slightly larger than usual and appears 

 homogeneous, b. the nucleus has increased in size, stains very darkly, lies at one end 

 and is surrounded by a clear space. c. the nucleus begins to show a darker spot 

 within it. d. two darkly- staining thread-like structures are present, e. the nucleus (?) 

 does not stain so deeply, and is not siirrounded by a clear space but has a small 

 amount of protoplasm clinging to it; within it at the somewhat pointed extremity can 

 be seen a light line indicating the larger terminal part of the thread, f. the wall 

 of the thread-cell and the thread itself are clear. 9. The fully developed thread-cell. 



PLATE III. 



Fig. 4. Portion of a small branch with the zooids expanded. The skeleton is 

 covered by the soft parts but the dark lines indicate the external parts of the chitinous 

 network which show through the soft structures. 



