NOMENCLATURE. 7 



of the spur. The part called the spur is often so broad, that 

 the name becomes not very appropriate. The angles are 

 denominated, from the adjoining margins, as basi-carinal, 

 or basi-scutal angle, &c. On the under side, in the upper 

 part, there is an articular ridge, and on its scutal side, an 

 articular furroio, receiving the articular ridge of the scutum. 

 In the basi-carinal corner of the valve there are often crests 

 for the attachment of the tergal depressor muscle. 



Sack, Body, Cirri, Mouth. — A slit-like orifice between 

 the opercular valves leads into the sack, in which the body is 

 lodged. The body consists of the six (perhaps the seven) 

 posterior thoracic segments of the archetype Crustacean ; the 

 first of these six segments (or first two, if there be seven 

 segments) is developed on its dorsal aspect into a part, 

 which I have called the prosoma* (see fig. 1, c, PI. 25). 

 There is no abdomen. The thoracic segments support six 

 pairs of cirri. Each cirrus consists of a two-jointed pedicel, 

 carrying two multi-articulated rami. Rarely there are 

 articulated caudal appendages {appendices candales) on each 

 side of the anus. The prominent mouth consists of a 

 labrum, palpi, mandibles, maxillce, and outer maxillce, the 

 latter resembling a lower lip : these organs may be conve- 

 niently spoken of, after Milne Edwards, as gnat Idles . 

 Within the sack, attached to its carino-lateral end, a folded 

 membrane forms the branchice. The sheets of ova lying 

 within the sack are called the ovigerous lamella. 



I have often found it convenient to designate the mem- 

 brane investing the body, lining the sack, &c, by its proper 

 chemical name of c/iitine, instead of by horny, or other 

 such equivalents ; but when covering parts of the shell, for 

 brevity's sake I have often spoken, of it as an epidermis, but 

 I do not believe that such is its nature. When this mem- 

 brane sends into the body of the animal rigid projections or 

 crests, for the attachment of muscles or any other purpose, 

 I call them, after Audouin, apodemes. For the underlying 

 true skin, I use the term corium, 



Relative position of parts. — The centre of the generally 

 flat basis, which is cemented to the supporting surface, is 



* A discussion on the homologies of the different parts is given under the 

 head of the Metamorphoses of the Balanidse. 



