370 



LOWER INVERTEBRATES. 



Tlie species known as Parasira catenulata occurs on both sides of the Atlantic, 

 specimens being most numerous in the West Indies and the Mediterranean. A few 

 years ago a single specimen was found on the southern New England coast. Two 

 species of the genus have been described, but, according to Steenstrup, the differences 

 are sexual, the form known as P. carena being the male. The flesh of this species is 

 tough and unwholesome. The ventral surface of the body is ornamented by tubercles 

 and reticulating ridges. Closely allied is the genus to which Professor Verrill has 

 applied the name Alloposus, but which at the same time presents resemblances to the 

 last family in the membrane which unites the arms for about two thirds their length. 

 Large female specimens of the only species (A. mollis) weigh over twenty pounds, and 

 have a total length of thirty-two inches. 



The only other genus of the family which we need to mention is Argonauta, the 

 one which embraces the paper sailors, so well known for the delicate and beautiful 

 shell which they secrete. This shell, however, is not homologous with that of other 



molluscs, as is seen by the method 

 of its formation. In the female the 

 two dorsal arms are expanded into 

 two broad membranes <it their ex- 

 tremities, and it is these two mem- 

 branes that secrete the shell. The 

 purpose of the shell is merely to 

 protect the eggs, and, although'the 

 female sits in it, she has no organic 

 connection with it. This fact, taken 

 with the finding of female paper 

 sailors without shells, led to many 

 a dispute among the older natural- 

 ists, and for a long time it was 

 maintained that Argonautu took 

 the shell of some other animal. These delicate egg-nests, which are produced by the 

 females alone, are frequently washed ashore in tropical countries in large quantities. 

 On our shores they are rare, only two with the animal having been reported, though 

 about a dozen dead shells have been dredged by the XT. S. Fish Commission a hundred 

 miles south of the New England coast. 



The paper sailor is often described at the present day as taking advantage of fine 

 weather and coming to the surface, when it lifts its broad arms and sails away before 

 the breeze. It does nothing of the kind ; it swims, as do all other cephalopods, by 

 forcing water through its siphon, its broad arms clasping the shell and the others trail- 

 ing, behind. The ArgonoMtce, of which nine species have been described, are all pelagic, 

 coming to the surface at the spawning season, and sinking to the bottom at other 

 times. The male is but about an inch in length, being sometimes scarce a tenth of the 

 size of the female. 



Tlie OcTOPODiD^ are more littoral than the last family, and have the mantle con- 

 nected to the visceral sac by muscular bands, while there are no aquiferous pores in 

 the head ; the arms are long and more or less webbed, and are furnished with one, 

 two, or three rows of acetabula, and lateral fins may be present or absent. 



First in order comes the Octopus of science, the subject of many a weird and blood- 

 curdling tale. Most prominent among these is that desci-iption of the devil-fish given 



Fig. 488. — SlieU or egg-nest of Arganauta argo, the paper sailor. 



