70 SYNOPTIC COLLECTION. 



arranged in one layer. The ectoderm is colored blue in 

 the figure, and is seen to invest the whole body and cover 

 the projecting spicules. 



Ascetta represents the group of sponges known as 

 Ascones. The " canal system " of other sponges scarcely 

 exists in this group, since the body cavity is a sac or am- 

 pulla without radiating canals. If we imagine a number 

 of Ascetta-like forms budding from a common base and 

 from each other's sides, so as to form a bushy colony, we 

 have a sponge like Leucosolenia (No. 63), one of the 

 commonest on our coast. This is a simple thin-walled, 

 Calcareous sponge like Ascetta except that the young 

 single tube gives rise to branches by budding, and these 

 branches to others, until a colonial form is produced. PI. 

 63, figs. 1-3, show the structure of the adult Leucosolenia 

 (species probably coriacea Montague). A character of 

 this genus is the sieve which extends over the cloacal 

 opening seen in fig. i, where a portion of the upper part 

 of the tube has been cut away. Fig. 2 is a vertical sec- 

 tion of one tube showing the flagellate cells of the endo- 

 derm, the large central cavity, the ectoderm, and at the 

 top the sieve. Rising above the sieve, the ectoderm by 

 doubling upon itself forms a two layered ectodermal 

 collar. In fig. 3 the sieve is separated from the tube. 

 Its cells have a central portion containing a nucleus 

 which is more clearly seen in one of the upper and cen- 

 tral cells, where it is indicated by a black circle. The 

 cells extend out into a number of processes and unite 

 with those of other cells, thus forming a network with 

 large openings. In this case the body of the cell forms 

 the node, but sometimes the node is produced by the 

 union of three cell processes. Fig. 4 is a spicule of this 

 genus. Fig. 5 is another species of Leucosolenia (Z. 

 dathrus O. S.) which has been described as without large 

 openings. When seen in healthy living condition the 

 cloaca is widely extended (fig. 6) ; when contracted the 

 opening closes as in fig. 7. The sphincter by which the 



