VII MOLLUSCA—HEAD 103 



with such proboscides are nearly all carnivorous (the Tritonidcc, Doliidoe, and Casai- 

 didce, among the Stenoglossa the Eachiglossa, and a number of Toxiglossa). 



Most male Monotocardia have a non-invaginable penis, which varies in shape, on 

 the right (rarely on the left) side of the head or neck, near the tentacle ; this organ 

 in most cases belongs morphologically to the foot, being innei'vated from the pedal 

 ganglion ; less frequently it is a cephalic appendage, and is then innervated from the 

 cerebral ganglion (Fig. 71, p. 73). 



The head of the Hetcropoda carries two tentacles (occasionally rudimentary : Ptero- 

 trachea, Firoloidea). The eyes are sessile or placed on small prominences near the 

 bases of the tentacles on their outer posterior sides. That part of the head which 

 lies in front of the tentacles is jirolonged to form a large proboscis-like non-invagin- 

 able snout. 



2. Opisthobranchia. 



The shape of the head in this order varies to an extraordinary degree, and can 

 here be only generally described. It usually carries two pairs of tentacles ; the 

 posterior pair, which are called rhinophores, are perhaps olfactory. Their surface is 

 often increased by the formation of circular folds. They frequently rise from the base 

 of pits into which they can be withdrawn. The head is rarely prolonged into a 

 proboscidial snout. The eyes are sessile. 



Among the Tectibranchia, the Cephalaspidm are distinguished by peculiarities of 

 the head. It carries dorsally a flat fleshy disc, the cephalic or tentacular disc 

 (Fig. 14, p. 10), which is regarded as the result of the fusion of the tentacles, and 

 which, by its shape, recalls the propodium of the Natkida: or Olividce among the 

 Prosohranchia. This cephalic disc carries the sessile eyes on its dorsal side, and its 

 posterior lobe, which is sometimes jiroduced in the shape of two lateral tentacular 

 processes, shifts about over the anterior portion of the shell. The shape of this disc 

 varies considerably in details. 



Of the very numerous Nudibranchia we shall only notice two extreme forms : 

 Tethys and Phyllirrhoe. 



In Tethys, the head takes the form of a large fiat disc, almost semicircular in 

 shape and fringed at the edge ; this carries on its upper surface two conical rhino- 

 phores, which can be retracted into large sheaths. 



In Phyllirho'e (Fig. 19, p. 12), the head is produced into a short proboscidial 

 snout, which carries only two veiy long curved tentacles ; the bases of these are 

 encircled by integumental folds, and they may be considered as rhinophores. 



Pteropoda gymnosomata. — The head is distinct, and carries two j^airs of 

 tentacles, one labial and the other nuchal. The fonner answers to the anterior, and 

 the latter to the posterior tentacles or rhinophores of the Tectihranchia, especially 

 those of the Aplysiidce. The nuchal tentacles are generally small or rudimentary, 

 the rudiments of the eyes lying at their bases. 



Nearly all the Gymnosmnata, as highly-developed carnivorous animals, ai-e provided 

 with a proboscidial snout which, commencing at its tip, can be completely invaginated, 

 and carries at its base, when evaginated, buccal appendages innervated from the 

 cerebral ganglion. 



Definite compensatory relations exist between the proboscidial snout and the 

 buccal appendages : — 



1. When the proboscis is specially long, the buccal appendages are wanting 

 {Clionopsis). 



2. When the proboscis is of median length, it carries suckers at its base, or a 

 pair of long appendages provided with suckers {Pncumodermido}, Fig. 76, p. 79). 



3. When the proboscis is short, there are long anterior tentacles, and at the base 



