12 
THE VOYAGE OF H.M.S. CHALLENGER. 
The choanocytes are well preserved ; all the details of their well-known structure, 
except the collar, being clearly displayed. The fenestrated membrane is preserved in 
some of the chambers only, and when present the flagella are not visible. On the other 
hand, when the flagella are preserved the fenestrated membrane cannot be traced. The 
length of the body of a collencyde is about 0'004 mm., of the flagellum 0’016 mm. 
The mesoderm of the choanosome is a collenchyma ; it varies considerably in relative 
abundance, amongst the chambers it is frequently very poorly developed, the walls of 
these often lying nearly in contact with each other, but in other places it attains 
considerable thickness, and is subject to extensive modification ; thus it forms a proper 
wall to the water canals, which are furnished with vela, usually provided with sphinctrate 
apertures, and it follows the spicular fibres in their course, and then contains numerous 
fusiform cells which usually lie parallel to the length of the spicules. 
Genital products. — Sperm masses are abundantly present, occupying cavities in the 
mesoderm. They arise from finely granular cells, about 0‘018 mm. in diameter, with 
an evident oval nucleus 0‘007 mm. in diameter, and a spherical nucleolus 0‘003 mm. 
in diameter; similar but smaller cells occur in the mesoderm, down to 0'008 mm. in 
diameter. By segmentation of the mother cell the sperm mass is produced. It is not 
enclosed in a cover-cell, but the cavity of the mesoderm in which it lies is lined by 
so-called endothelium, surrounding which is the usual collenchyma, without any special 
accumulation of collencytes. The developing sperm clusters occur in cavities of from 
0‘04 to 0‘05 mm. in diameter, but the mature sperm occupies cavities of much larger size, 
from O’l to 0T75 mm. in diameter. The ripe spermatozoa present an oval head, about 
O'OOl mm. long, from the side of which, near one end, the “tail” arises and attains a 
length of about O'Ol mm. or more. 
Skeleton. — The spicular fibres are spirally arranged, in a manner which will be found 
more precisely described under the species Cinachyra harhata, where a similar arrangement 
obtains. 
Spicules. — The tri£enes begin to appear within the choanosome, just below the 
ectosome ; the youngest forms are found at a distance of about 2 ‘25 mm. below the outer 
surface, and between these and the exterior a series of cladomes of gradually increasing 
size occur. 
The cladomes of the triaenes lie on the exterior of the spicular fibre, and it would appear 
to be owing to the pressure exerted against them, when they come in contact with other 
spicules, that one or more cladi are sometimes suppressed, or, if not suppressed, forced 
into an irregular mode of growth, by which they become deformed, one or more cladi of 
an anatrisene, for instance, pointing forwards instead of backwards, or, if retaining their 
usual direction, losing the usual regular curve and becoming crooked. 
Associated with these variously deformed spicules are others in which a fourth cladus 
is developed, but this more frequently happens with the anatrisenes than the protrisenes. 
