Aspidobranchia.] GASTROPODA. 61 



ORDER 1. ASPIDOBRANCHIA. 



(= Diolocardes ; Scutibranchia.) 



These are Streptoneura in which, the nervous system is still but 

 little concentrated. The pedal centres have the form of long gang- 

 lionated cords, to the anterior end of which the pleural centres are 

 attached ; the cerebral ganglia are widely separated from one another, 

 and are united by a long commissure lying in front of the buccal mass 

 and the salivary glands. The eye is open, or, if closed, has a very 

 small pellucida. The central teeth of the radula are multiplied. 

 Ctenidia are almost always present ; they are bipectinate, and free at 

 their distal ends. As a rule, the Aspidobranchs exhibit well-marked 

 traces of the original bilateral symmetry, having two auricles to the 

 heart, and two kidneys (one only in the Neritacea). 



The order Aspidobranchia includes the most archaic Gastropods ; 

 it includes two suborders, the Docorjlossa and Rhipidoglossa. 



SUBORDER 1. DOCOGLOSSA. 



(= Onychoglossa, 0. Sars.) 



The organs of respiration are represented either by a ring of laminae 

 (secondary or pallial gills) beneath the mantle-margin, or by a comb- 

 shaped trae gill in front, anterior to the heart, or by both true and 

 secondary gills. The eyes are open, and devoid of a crystalline lens. 

 There are two osphradia, but neither hypobranchial glands nor oper- 

 culum. The radula is usually very long, bearing vertically elongated 

 beam-shaped and hooked teeth, and there are at most 3 marginal 

 teeth on either side. The heart has only 1 auricle, and neither it 

 nor the pericardium are traversed by the rectum. The visceral mass 

 is cone-shaped, without a spire. Shell symmetrical, non-spiral, conic 

 or bowl-shaped. Cambrian to Recent. The suborder includes about 

 1,400 species. 



Vernacular name. Limpets. 



Maori. Ngakihi. 



Fam. ACM.JEIDJS, Philippi. 



; Animal with a single bipectinate ctendium on the left side, iree 

 for the greater part of its extent. Radula with 1 central tooth on 

 each side, 2 lateral teeth (sometimes absent), and sometimes with 1 or 

 2 marginal teeth. 



Shell patelliform, conical, the apex more or less anterior, the em- 

 bryonic shell conical, not spiral. Cretaceous to Recent. 



They live on rocks and seaweeds, generally at very moderate 

 depths. 



