376 LOWER FORMS OF MARINE LIFE 



some of the most highly prized treasures of the collector. These beautiful 'shells 

 are, however, borne by the females alone, and are detachable, and employed solely 

 as receptacles for the eggs, so that they are in no wise comparable to the shells of 

 a nautilus or a spirula, which form an essential part of those molluscs. 



Among cuttle-fishes, one of the most notable is the octopus {Polypus, or Octopus, 

 vulgaris), of which the disc may grow to eight or ten feet in diameter, although 

 ordinary specimens are much smaller. Some years ago swarms of octopus ap- 

 peared on both sides of the English Channel and inflicted untold harm on the 

 crab, lobster, and oyster fisheries. The increase in the numbers of these com- 

 paratively scarce cephalopods was first noticed in the spring of 1899, since which 

 date they appeared in such hosts as to justify the application of the term 'plague' 

 to the visitation. The quantity of shell-fish destroyed by these voracious 

 cephalopods is almost incredible. Fortunately the octopus itself is not difficult 

 to capture, and a fisherman can easily account for more than half a hundred per 

 week in the course of his work. On the French coast they are taken and sold 

 for food. The species is figured in the chapter on the Lower Forms of Marine 

 Life in the Northern Seas, on page 303 of the second volume. 



The genus Architeuthis is noteworthy on account of including the largest 



known representatives of the entire molluscan class ; one specimen, of which a 



cast is exhibited in the British Museum, measuring no less than thirty feet to 



the extremities of the single pair of elongated arms with which this frightful 



creature first seizes its prey. From the evidence of fragments of the limbs this 



species must, however, be a dwarf to some of those inhabiting the ocean depths. 



The aforesaid species, A. princeps, has a wide distribution, and has been taken 



off the coast of Ireland. Ordinarily the elongated pair of arms are kept drawn 



back, but when prey is in sight they are shot suddenly forward, seize the victim 



with their suckers, and draw it back to be done to death by means of the shorter 



arms and the cruel, parrot-like beak. 



A certain number of animals are a source of trouble to 

 Arrow- Worm. . 



naturalists on account of the difficulty of assigning them to their 



proper systematic positions. Among these, is the marine creature popularly 



known, from its shape, as the arrow-worm, and scientifically as Sagitta. In 



text-books these curious organisms, for there is more than one species, are classed 



with the worms, but there was an old idea that their real position is among the 



molluscs. This view has been revived and strengthened by an English naturalist, 



who places arrow-worms in the Mollusca, next the nautilus and cuttle-fishes. 



Univalve Although shell-collecting is by no means such a fashionable 



Molluscs, hobby as was at one time the case, few natural objects are of 



greater beauty and interest than the shells of many of the species of univalve, or 



gastropod, molluscs. Among the smaller kinds, reference may first be made to 



the beautiful wentle-traps (Scalaria) and violet-snails (lanthina). In the former 



the white, turriculated shell is ornamented with bold vertical ribs, while in the latter 



it is snail-like, with the under surface of a rich violet. These violet-snails are 



pelagic in habit, and in order to enable them to float on the surface, they construct 



a light raft of air-bubbles, on which the eggs are also carried. In consequence of 



this pelagic habit, violet-snails are so widely distributed that, although their true 



