REPORT ON THE TETRACTINELLIDA. 
137 
Astvella vosmaeri measures 30 mm. in height, 45 mm. in width, by 41 in breadth ; the 
smaller is about 35 mm. long by 23 mm. wide. In neither, after a careful search, could 
any trace of an oscule be discovered. The surfaces of both are worn nearly smooth, as if 
the sponges had been rolled about on the sea-bottom, the hispidating spicules remaining 
only where protected by the attached sponge and at the opposite pole. The pores measure 
from 0‘025 to 0’0775 mm. in diameter, they occur in sieves which overlie the ends of the 
canals forming the chones. The chones (PI. XL. fig. 8) consist of a canal which sends off 
numerous branches towards the dermal membrane, each branch terminating beneath a 
pore-sieve. A single chone thus receives water from several pore-sieves, and may drain 
an area I "5 mm. in diameter. In some cases, however, only one or two branches are 
given off from the main canal. In the other direction the canal is continued inwards 
through the fibrous layer of the cortex with a very much diminished diameter; it is easily 
traced in thick slices by the layer of scattered pycnasters which accompany it, and which 
extend over the face of the sphinctral muscle which closes its inner end. This sphincter 
forms a ring-like swelhng on the inner face of the cortex around the end of the chonal 
canal. 
Ectosome . — The cortex consists of a thin outer layer of fibrous tissue, 0‘06 mm. in 
thickness, covered by the external epithelium and its underlying layer of pycnasters ; of 
a thick layer of clear collenchyma, 1 "0 to 1 *5 mm. in. thickness, containing, in addition to 
a network of coUencytes, elongated fusiform cells, which, however, are not numerous 
enough to give it a fibrous appearance ; and finally of an inner layer of fibrous tissue, the 
component fusiform cells of which are arranged in the usual way, i.e,, in fibres which run 
concentric with the surface in various azimuths ; this layer varies from 0T6 to 0'475 mm. 
in thickness, and the total thickness of the cortex varies from 1‘35 to 2‘0 mm. Where 
the megascleres pass through the cortex the inner fibrous layer is produced outwards into 
conical extensions, which surround the cladal ends of the rhabdomes ; a single rhabdome 
with the attached fusiform cells extending obliquely downwards from it on all sides into 
the fibrous layer may frequently be observed. 
Choanosome . — The mesoderm is a sarcenchyma, amidst which fusiform cells are some- 
times seen wandering, especially in the neighbourhood of the canals. The chones lead 
into subcortical crypts, which, like the larger excurrent canals, are furnished with coUen- 
chymatous walls, in which, in addition to the plexus of collencytes, deeply stained 
fusiform cells, and rarely large granular cells elongated in the same direction as the 
fusiform cells, are to be seen. Although no discoverable oscules exist, there is no diffi- 
culty in distinguishing the incurrent from the excurrent canals, the interdigitation of the 
two systems being very clearly revealed in good sections. The large excurrent canals, 
with collenchymatous walls, communicate with the fiagellated chambers only by their 
terminal branches, and never directly through the walls ; indeed, branch canals may some- 
times be observed extending between the surrounding sarcenchyma and the collenchyma 
(ZOOL. CHALL. EXP. — PART LXIII. — 1887.) ElT 18 
