NUDIBKAXCHIATA. 17 



Order NUDIBRANCHIATA. 



Gills exposed, or contractile into cavities on the surface of 

 the mantle. Adult animal without any shell. Larva shell- 

 bearing. Foot elongate, formed for walking. Sexes united. 



"While the numerous tribes of Mollusks furnished with 

 testaceous coverings offer us objects of contemplation 

 remarkable alike for their extreme beauty and the durability 

 of their calcareous envelopes, the scarcely less extensive 

 and certainly far less known families of Naked-gilled Gaste- 

 ropods exhibit an astonishing variety of form, extreme deli- 

 cacy of organisation, and great diversity of colour to capti- 

 vate the eye and occupy the attention of those who wander 

 by the shore or explore the depths of ocean. Clinging to the 

 stems of floating sea-weeds, many, like the Anthobranchs, will 

 be seen extruding their flower-like gills of surpassing ele- 

 gance, exploring with their foliated tentacles or complex 

 mantle-filaments the plants around them, the brilliant hues 

 of their striped or spotted bodies glancing through the 

 water; some will be observed with bodies so fragile and 

 pellucid that you may see the colour of their blood and 

 count the pulsations of their hearts ; some will be seen to 

 have their gills disposed in rows of papillary tubercles on 

 the sides of their bodies, like the iEolids, or tree-like and 

 branching, like the Tritonias ; the foreheads of some will be 

 smooth and simple, while those of others will be found 

 adorned with various singular appendages ; in others, again, 

 all processes will disappear, all branchial arrangements 

 vanish, and we shall meet with forms almost as simple in 

 their structure as the Nemertoid types among the Annelids. 



