xii PHYLUM MOLLUSCA 713 



the testis. narrower and straight distally ; it opens in front in the 

 mantle-cavity into the proximal end of tfre sperm-groove, which, 

 as already mentioned, runs forwards along the right side and 

 becomes continuous with the groove traversing the penis. The 

 oviduct (Fig. 623, ovid.) is proximally a very delicate tube with 

 colourless, transparent walls. This runs forwards to the right 

 side of the mantle-cavity, where it assumes the character of a stout 

 tube (ovid'.) with thickened glandular walls, which passes forwards 

 close to and parallel with the rectum, and opens on the exterior 

 near the anus. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Gastropoda are unsymmetrical Mollusca, with a mantle 

 which is not divided into two lateral portions, and usually a shell, 

 which does not consist of two lateral valves, but of a single, 

 unsymmetrical, usually spirally coiled valve, enclosing a visceral 

 mass of corresponding form. There are, typically, two plume-like 

 ctenidia enclosed in a mantle-cavity, but there may be only one ; 

 and in air-breathing forms ctenidia are not developed, respiration 

 taking place through the wall of the mantle-cavity itself. A 

 distinct head bearing eyes and tentacles is present in the majority. 

 The foot is situated behind the head, and usually has an extensive 

 flattened ventral surface. The buccal cavity contains an odonto- 

 phore. The kidney is usually single. The nervous system contains 

 distinct cerebral and pleural, besides pedal, visceral, abdominal, 

 and buccal ganglia. The sexes are sometimes separate, sometimes 

 united. The larva passes through trochophore and veliger stages. 



Sub-Class I.- Streptoneura. 



Gastropoda in which the visceral connectives are in most cases 

 twisted into a figure of 8, and in which the sexes are distinct. 



ORDER 1. ASPIDOBRANCHIA. 



Streptoneura with the nervous system but little concentrated : 

 the pedal ganglia are produced into long cords with the anterior 

 ends of which the pleural ganglia are fused ; the cerebral ganglia 

 wide apart ; the osphradium little developed. There is nearly 

 always a single ctenidium or a pair, plume-like and free distally. 

 The auricles and the kidneys are usually paired. 



Sub-Order 1. Docoglossa. 



Aspidobranchia in which the pleural ganglia are not connected 

 with the opposite visceral connective. The eye is in the form of 

 an open pit, without lens. There are two osphradia, a single jaw, 

 and no operculum. The visceral mass is conical. 



This section includes the Limpets (Patellidce). 



VOL. i z z 



