284 LOWER INVERTEBRATES. 



line, or, as in the Teredines, 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 protrusion of the short and truncated foot. 

 At the other end it is prolonged into a very large siphon, which, in the Teredos., 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 

 siphon. 



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

 in submerged 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 comparati\ely 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 

 Hatschek, from which we learn that, like other 

 molluscs, it passes through a veliger stage. Soon 

 after this the young larva comes across a piece 

 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 

 Fig. 312.— Young Tercrfo before it begins excavations which are to result in that prison, 



its burrow. . ^ 



which It never leaves until death. Exactly how 

 it excavates is still a matter of dispute, 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 young 

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

 shelly tube, which, 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 communica- 

 tion with the external world being 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 through the excurrent sijjhon. The food which nourishes the animal is, here 

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

 animals and plants. Notwithstanding 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 gi-ain, 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 spring 



