CHITONID^. 203 



with Dr. Bowerbank in his account of the composition 

 of univalve shells^ as evincing a definite organic arrange- 

 ment and not a simple crystallization. 



Order I. CYCLOBRANCHIATA. 

 Family CHITO'NID^, Guilding. 



Body oval, oblong, or elongated, semicyHndrical, rounded at 

 each end : mantle thick, covering the back, and encircling the 

 sides with a girdle which is free at its edges : head sessile, 

 surmounted by a membranous veil or hood, and containing a 

 pair of horny jaws and the front of a long and slender tongue 

 bristling with numerous teeth, which extends into the interior 

 of the body, and is folded up within it : no tentacles or eyes : 

 gills forming a row of small pyramids on each side, which 

 meet behind the head, lying between the mantle and the foot, 

 and extending from behind to the front : foot muscular, occu- 

 pying the whole of the under surface : vent or excretory duct 

 placed opposite to the head at the end of the foot. 



Shell composed of separate arched plates, which are inserted 

 in the mantle along the back breadthwise ; they are usually 

 external. 



I am not surprised at Lamarck calling this a singular 

 and strange group^ nor that there has been such difficulty 

 in assigning to it a definite place among the Inverte- 

 brata. In the larval state they resemble Isopodous 

 Crustaceans^ or they might even be mistaken for tiny 

 Trilobites ; and the adult may be compared to Onisci de- 

 prived of antennse, eyes, and feet. They are also not unlike 

 species oiAphrodita. When a boy I was cruelly deceived 

 in thinking that I had found a huge and new Chiton, 

 having got hold of a Sea-mouse in the sand at low water. 

 De Blainville believed that their natural affinities lie 

 with the Annelids, and he raised them to a tribal rank 

 under the name of Polyplaxiphora. The circulatory 



