360 LOWER INVERTEBRATES. 



ists are not in the habit of trailing the surface net in tlie winter. Tliis species reaches 

 a length of about an inch and a half. 



In the PNEUMODERMONiDyE the body has much the same shape as in the other, but 

 it is distinguished by the presence of posterior external gills, and two extensible arms 

 bearing suckers much like those of the cephalopods. Specimens of P nemnodermon 

 are common in the Mediterranean and in the warmer Atlantic. They swim strongly, 

 but when touclied by a foreign object they roll themselves up like an armadillo, and, 

 feigning death, sink until out of the reach of ajiparent danger. 



Class III. — CEPHALOPODA. 



We have now reached the highest group of the molluscs, the one which embraces 

 the squids and cuttle-fishes, and which has given rise to many of tlie tales and legends 

 of the sea-serpent and other marine monsters. But before indulging in these stories, 

 we must first look at some of the structural and embryological features which mark 

 off the cephalopods from the rest of the molluscs, a distinction which has always been 

 recognized, though the reasons therefor have not until recently been scientifically 

 formulated. This confusion has been due to the fact that the cephalopods are a higlily 

 organized type, which, in course of a long residence on the earth, have eliminated all 

 traces of a metamorphosis from their development, and which consequently j)rogress 

 in a straight line from the egg to the adult. 



The body of the cephalopods is enveloped in a mantle, open only at the anterior 

 end, from whence proceeds the head, separated from the body by a slight constriction 

 or neck. On either side of the head is a large eye, the structure of which will be 

 described farther on. Beyond the eyes arises a circle of usually eight or ten lobes, or 

 'arms,' in the centre of which is the mouth. Part of the difficulty in homologizing the 

 cephalopods with the other gasteropods has arisen in connection with these arms. 

 They were early recognized as corresponding to the foot, and, as they arose from the 

 head, the name Cephalopoda (head-footed) was applied to the group. At a later date, 

 the siphon (a structure to be described immediately) was studied, and again the foot 

 was recognized. Which was right? Further investigation showed that both were 

 correct. The arms correspond to the anterior division (pi'opodium), the siphon to the 

 middle one (niesopodium), while the metapodium is absent, or represented by a rudi- 

 ment, forming a v.alve in the siphon of some species. 



The .arms, or tentacular lobes, as we have just said, surround the mouth. In the 

 Nautilus they bear peculiar tentacles, capable of being retracted into tubular sheaths. 

 In the rest of the cephalojjods they are longer, and are armed on one side with spheri- 

 cal sucking organs, — acetabula, they are called. Each one of these sucking organs 

 has a rim, which can be closely apjilied to any foreign body, and the hold increased 

 by means of numerous fine hooks around the edge. In the centre is a moveable por- 

 tion, which, from its jihysiological resemblance to the plunger of a pump, has received 

 the name of piston. The animal ajiplies these suckers to any foreign object, and then 

 pulls out the piston. Besides these suckers, certain arms of some species bear numbers 

 of sliarp, recuiwed hooks. Of the manner of capturing the prey we shall speak later. 



INIorphology shows (in a way which need not here be discussed) that these arms 

 and tentacles correspond to the first division of the foot (propodium) of other mol- 

 luscs. Next comes the siphon, the homological niesopodium. Tliis is a tubular 

 structure, arising from what is usually called the ventral surface of the body, and 



