318 Dr Coldstream 07i the Structure and 



1811, as a " new and highly interesting species,*" sent to him 

 *' through the politeness of his attentive and worthy friend, Ro- 

 bert Stevenson, Esq.,*" and stated, that " it occurs in the great- 

 est abundance at the Bell-Rock;, in the old wood- work, used whilst 

 the light-house was building, which it perforates in a most alarm- 

 ing manner, entering to the depth of two inches or more, boring 

 in every direction." But, although the characters and general 

 description of the animal given by Dr Leach have been copied 

 by many systematic writers, I am not aware of any other natu- 

 ralist having given to the public the results of extended inqui- 

 ries concerning it. I shall therefore state all the facts relating 

 to its structure and habits, which I have succeeded in ascertain- 

 ing satisfactorily, being convinced that we ought to know, as 

 well as possible, every thing connected with the natural history 

 of an animal, which, although so minute, exerts so powerful an 

 agency with regard* to us. But I am far from supposing that 

 I have exhausted the subject. There are many points in its 

 structure which I cannot make out satisfactorily, and 1 have 

 wanted opportunities of ascertaining what are its habits in certain 

 circumstances, and the modes in which it exercises some of its 

 functions. 



I. External Characters, 



The average length is ^^^o^gths of an inch, and the breadth 

 ^Jgths ; but several individuals occur /gthsof an inch in length, 

 and of proportionate breadth. The female is about one-third 

 larger than the male. The usual colour of the dorsal surface 

 is a pale greyish-brown; some individuals are streaked about 

 the head with lines of a darker shade. The legs and ventral 

 surface of the body are almost white. The body is in general 

 translucent, but not so much so as to admit of the internal organs 

 being seen distinctly. The form is represented in Figs. 1. and 

 % (PL VI.) It is semicylindric,'convex above, slightly concave 

 beneath, rounded before and behind. The breadth being con- 

 ^derably less than the length, the general aspect is somewhat 

 vermiform. 



The whole animal is composed of fourteen segments, of which 

 CHQe forms the head, the seven next to it bear each a pair of legs, 

 and the remaining six constitute the tail. The head is rather 



