CHAP. IX. CYCLOBRANCH1A. TECTIBRANCHIA. 247 



have any degree of affinity with the Nudibranchia, 

 whose gills 'are invariahly exposed, and arranged sym- 

 metrically. All are marine, and crawl on their helly ; 

 but the major part, from having the lobes of the mantle 

 dilated, are likewise capable of swimming. Nevertheless, 

 they are thick, unsightly creatures, resembling, when 

 caught, shapeless lumps of flesh; for the tentacula, 

 where they exist, are short ; the shell (if present) is 

 hid in the mantle ; and it is difficult to know, at first, at 

 which extremity the head is placed. They cannot be 

 preserved effectually in spirits ; but by being placed in 

 sea water, when first caught, they soon begin to crawl, 

 and exhibit their true form. The annexed sketch of our 

 Aplysia Sicula (fig. 45.), taken from the living animal, 



will give a good idea of that genus : it has the power, 

 like many other species, of emitting a copious black or 

 purple fluid when caught, like some of their prototypes 

 the Cephalopoda, for the purpose of discolouring the 

 water, and eluding their enemies. 



(231.) The TECTIBRANCHIA, even from the little 

 yet known of them, appear to form a most natural 

 group, in which the circular succession of the five 

 types, and the analogies they bear to the conterminous 

 tribes, may be distinctly traced. The variation in the 

 form and position of the branchia, although quite in- 

 sufficient to remove the aberrant types from Aplysia, is 

 yet of much importance in determining the primary 

 divisions. Thus, in the JButtinas and the Aplysiance, the 

 B 4 



