284 LOWER INVERTEBRATES. 



liiK', or, as in the Teredina", are borne at the extremity of the long, calcareous tube 

 formed by these animals. The margins of the mantle are almost completely united, 

 leaving only a small opening in front for the jjrotrusion of the short and truncated foot. 

 At the other end it is prolonged into a very large siphon, which, in the Ih-edos, has 

 the power of secreting a calcareous tube. The gills are long and narrow, and, pos- 

 teriorly, are drawn out into a point which extends some distance into the excurrent 

 sipluin. 



They are all boring animals, and make their burrows, some in mud or sand, some 

 in submei'ged wood, while others bore into rocks, shells, or corals, at times doing con- 

 siderable damage to human interests. The distinctions between the two sub-families 

 is sufficiently emphasized in the foregoing paragraph. 



The genus Teredo, with about twenty-five so-called species, has gained a some- 

 what extensive notoriety under the popular name of ship-worm, and hence deserves 

 some little attention at our hands. The Teredo is a long, worm-like animal, bearing 

 at the larger end a comparatively small bivalve shell, while near the other are the 

 two accessory pieces, the so-called pallets, beyond which extend the separate ex- 

 tremities of the siphonal tubes. The development of the Teredo has been made the 



subject of exhaustive papers by Quatrefages and 

 'I), Ilatschek, from which we learn that, like other 



molluscs, it passes through a veliger stage. Soon 

 after this the young larva comes across a ])iece 

 ^ of submerged wood, or, in case it does not, it 

 dies. At first it creeps over the surface of the 

 timber, but soon it settles down and begins the 

 Mt begins excavations which are to result in that prison, 

 which it never leaves until death. Exactly how 

 it excavates is still a matter of disjiutc, but it seems probable that it is partly by 

 means of the edges of the pallets. Another theory of action is that the foot, with its 

 thick coriaceous epidermis, cuts away the wood. The hole made at first by the joung 

 Teredo is minute, about as large as a pin's head, but, once within the wood, it grows 

 rapidly, and its burrow is enlarged in the same proportion. As it excavates farther 

 and faither into the wood, it lines its channel by a calcareous deposit, thus forming a 

 shelly tidie, wliich, on our coasts rarely exceeds ten inches in length and a quarter 

 of an inch in diameter, but in favored localities some species attain a length of 

 two feet and a half. In this tube the .animal lives, its only means of conmnmica- 

 tion with the external world lieing through the small hole by which it entered the 

 timber. 



The Teredo does not feed upon the wood, the small particles which it erodes being 

 passed out thi-ough the excurrent sijAon. The food which nourishes the animal is, here 

 as elsewhere, brought in through the incurrent siphon, and consists of microscopic 

 animals and plants. N'otwithstanding the fact that the Teredo does not eat the wood, 

 the damage it does is very great. It was first brought prominently into notice at the 

 beginning of the eighteenth century, when, by its ravages in the piles and other sub- 

 merged wood which supported the dikes and sea-walls of Holland, it seriously threat- 

 ened the safety of that country. Hundreds of individuals will obtain entrance to the 

 same bit of timber, and, boring either with or across the grain, they soon convert it into 

 a mere shell, ready to break down at the slightest strain. The rapidity with which 

 they work is well illustrated by a fact recorded by Quatrefages. In the early spiing 



