108 RELATIONS OF MAN WITH EXTERNAL NATURE. 



trary, it is generally broad in proportion to its length, and belongs to what 

 has been termed the massive type. They have no external hairs, feathers, 

 scales, or horny or osseous plates ; their soft integument is commonly pro- 

 tected by an external calcareous shell composed of many layers, secreted, one 

 within the other, by the surface of a part of the body named the mantle. They 

 have no jointed limbs, but the locomotive organs are always soft and merely 

 musculo-cutaneous, usually forming the so-called foot or feet. The nervous 

 system consists of scattered ganglionic masses, commonly disposed in three 

 principal pairs, named, from their position or function, the cerebral, pedal, 

 and parieto-splanchnic ganglia. The cerebral ganglia, are very small in com- 

 parison with the brain of the Vertebrata, and there is no spinal cord. These 

 ganglia are joined together by commissural nerve-cords ; the cerebral ganglia, 

 placed above the oesophagus, are connected by two cords, which pass down on 

 the sides of the gullet, with the pedal ganglia, so that the gullet is inclosed or 

 surrounded by the anterior part of the nervous system, or passes through it ; 

 longitudinal commissures also connect the cerebral with the parieto-splanchnic 

 ganglia, but these latter and the pedal ganglia are not necessarily connected 

 by direct commissures. The alimentary canal is provided at the mouth with 

 soft non-segmented tentacles ; it lies, as in the Vertebrata, in a distinct cavity 

 named the perivisceral cavity, and is furnished with salivary glands and a 

 largely developed liver. The heart, dorsal in situation, contains two cavities, 

 and is connected with the systemic bloodvessels ; the . blood is corpusculated 

 but colorless. Most molluscs being aquatic, breathe by gills, but some terres- 

 trial genera possess pulmonary air-sacs. 



In the highest Mollusca, the cephalopoda or cuttle-fishes, there is frequently 

 found a rudimentary cranium supporting the cerebral ganglia, and sometimes 

 an internal horny or calcareous mass which may be taken to represent an in- 

 ternal skeleton. 



The Molluscous classes, Cephalopoda, Pteropoda, Pulmogasteropoda, and 

 Branchiogasteropoda, constitute a group named Odontophora, because they 

 possess a peculiar apparatus in the mouth, armed with teeth, and movable, 

 and sometimes called a tongue, but more appropriately the odontophore (Hux- 

 ley). The remaining class, Lamellibranchiata, form a group destitute of the 

 odontophore, and characterized by having a right and left bivalved shell and 

 two lamelliform gills on each -side of the body. 



In the various classes of Mollusca, different parts of the soft body and ap- 

 pendages, which have received different names, are variously developed. 

 Moreover, the alimentary canal follows such developments of the body, and 

 thus becomes not only elongated, but bent ; sometimes the concavity of the 

 bend is directed towards the abdominal or under surface, and, as the chief 

 nervous motor ganglia are placed on that side, such a bend is named a neural 

 flexure, as seen in the Cephalopoda, Pteropoda, and Pulmogasteropoda ; whilst 

 sometimes the concavity is turned towards the dorsal region, or in the direction 

 of the heart, and then it is termed a Immal flexure, as in the Branchiogaster- 

 opoda : in the Lamellibranchiata it is at first neural, but is afterwards spe- 

 cially modified. 



Molluscoida. The comparatively small subkingdom, Molluscoida, consists 

 of animals which, as implied by their name, have close relations with the 

 Mollusca, and were commonly classified with them ; but, as suggested by 

 Milne-Edwards, they may be more conveniently placed in a separate subking- 

 dom, being much simpler in their organization. The nervous system consists 

 of a chief ganglion with a few scattered ganglionic masses, or of a single gan- 

 glion only. The principal or single ganglion, as the case may be, is placed 

 close to the oral aperture or mouth, sometimes having a nervous cord around 

 the gullet. The alimentary canal is much flexed on itself, and sometimes is 

 only provided with one opening, viz., the mouth. When an outlet also is 

 present, it is placed, as a rule, near the mouth, the chief ganglion usually 

 being placed between the two openings. A heart is not always present, and 

 when it exists, is composed of but a single cavity. The oral aperture, except 

 in one group (Appendicularia) is always surrounded with numerous tentacles, 

 which are ciliated, and therefore probably respiratory, and are arranged either 

 in a circle, or else upon long arms, or upon a band or ridge shaped like a 



