STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 521 



Sponge is subject to considerable variation, but such strongly 

 marked deviations from the ordinary type as he figures have 

 not come under my notice, and I believe the essential 

 characters remain fairly constant. 



Certain rectangular marking on the surfaces of the Sponge, 

 due to the arraugement of the skeleton in the ectosome, are 

 very characteristic ; their nature will be best understood by 

 reference to fig. 8. 



One of the most obvious of the characters of the species is 

 the general flaccidity and emptiness of the body. Thus, 

 Bowerbank 1 remarks, "When divided longitudinally the parietes 

 of the Sponge did not exceed in the dried state the fourth of a 

 line in thickness at any part, and the internal cavity extended 

 the whole length of the Sponge. The greater number of them 

 were more or less in a compressed state, but in some there 

 were strong indications that this was due rather to collapse 

 than to natural form/' and Norman 2 begins his diagnosis of 

 the genus with the words " Sponge consisting of a single 

 clavate hollow body." I lay especial stress upon this character 

 as it will be found later on to be very intimately connected 

 with the arrangement of the canal system, and is one of those 

 characters in which the two genera, Rid lei a and Quasi 1- 

 lina, differ very markedly from one another. 1 may forestall 

 my account of the canal system so far as to state that the 

 hollow condition insisted upon by Bowerbank and Norman is 

 chiefly a post-mortem character, due to the shrinkage of the 

 loose and delicate choanosome. 



A full-grown specimen is usually less than an inch in height, 

 and less than half an inch in greatest width. Fig. 8 repre- 

 sents the Sponge of the natural size. 



Arrangement of the Skeleton. — This portion of my 

 subject has already been more or less fully treated of by 

 Bowerbank, Norman, and Vosmaer (locc. citt.), and, as I have 

 but little to add, I may dismiss it briefly. 



1 ' Mon. Brit. Spong.,' vol. ii, p. 64. 



' " Last Report on Dredging among the Shetland Isles," • Brit. Assoc. 

 Rep. for 1868/ p. 329. 



