STRUCTURE OF THE BARNACLE. 



21 



edges of the four lateral portions ; the general form of 

 the whole being compressed, and somewhat triangular. 

 At their exterior margin the lateral shells are cemented 

 together by membrane ; but not entirely, an open space 

 or fissure being left, through which the cirrhi, or jointed 

 arms of the enclosed animal, are protruded. At the 

 upper part of the shell, (regarding it as a whole,) the 

 membrane on which the different shelly pieces are de- 

 posited prolongs itself into a round stem, which in some 

 species is of the length of several inches ; by this the 

 animal is attached to rocks or other objects. In its 

 texture this stem is firm ; the outer skin is lined with 

 a layer of muscular fibres, running for the most part 

 in parallel longitudinal lines ; and by their action the 

 stem is capable of being twisted in various directions, 

 the animals thus having the limited power of altering 

 the position in which it may hang. Within the muscu- 

 lar tissue of the stem is a fluid secretion. 



In the genus tetralasmis, Cuvier, the stem is hairy, 

 and there are only four shelly valves. In the genus 

 2)ollicipes, Leach, besides the five valves, there are seve- 

 ral others at the commencement of the stem, generally 

 of small size, but sometimes nearly as large as the 

 principal valves. In the genus balanus, of which one 

 species is common on our shores, the tubular stem is 



