326 A MONOGRAPH O* 1 THE 



sponge immersed in water, and it is only when mounted 

 in Canada balsam that they can be distinctly seen in situ 

 in the. dermal membrane, on which they are not equally 

 dispersed, but occur in clusters at intervals. In the inter- 

 stitial membranes they are very few in number, and rarely 

 more than one occurs in the field at the same time. 



There is a considerable degree of resemblance in the 

 descriptions of I. Alderi and I. pawpera to that of I. 

 Edwardii, which may possibly lead to a doubt in the mind 

 of the student as to which of the three the specimen he 

 might have under examination belonged ; but a comparison 

 of the spicula alone of these three species readily distin- 

 guish each from the other. The length of the spicula of 

 I. Edwardii as compared with those of both I. pauper a 

 and I. Alderi is as seven to five, and although thus much 

 longer, the diameter is less than either of the spicula of the 

 other two species, and the form is much more purely 

 acuate. 



35. Isodictya lobata, Botoerbank. 



Sfongia lobata, Montagu. 



Sponge. Parasitical, coating Zoophytes, &c, branching 

 irregularly; branches short, lobate. Oscula simple, 

 dispersed, few in number. Pores inconspicuous. 

 Dermal membrane sparingly spiculous ; tension spi- 

 cula acuate, minute, and slender, dispersed, and some 

 the same as those of the skeleton. Skeleton. Sym- 

 metrical near the surface, diffuse, and irregular inter- 

 nally ; primary and secondary lines multispiculous ; 

 rete occasionally more than the length of one spiculum 

 wide. Spicula acuate, short, and stout. Interstitial 

 membranes. Tension spicula acuate, rarely cylindrical, 

 slender. Retentive spicula inequi-dentato-palmato- 

 anchorate and bihamate ; bows of both the latter 

 exter-umbonate. 



