CIRRIPEDIA. 157 



Balanoid of the whale, the compound shell is long, subcylindrical, 

 and wider above than below. The upper aperture is closed by an 

 operculum of two or more shelly plates. 



The animal in both sessile and pedunculate Cirripedes, is fixed to 

 the bottom of the shell with its head downwards. The head is there- 

 fore superior in the ordinary pendant position of the Barnacle. 



The movements of the peduncle in such species are effected by a 

 strong muscular tunic, as shown in these preparations. * The action 

 of these muscles is antagonised by the elasticity of the external horny 

 tunic. The common Barnacle approximates its large pair of valves 

 by a strong transverse adductor muscle ; the body or visceral mass 

 of the Barnacle is moved towards the aperture of the shell, which is 

 thereby at the same time widened by longitudinal muscular fibres, 

 and is retracted by shorter fibres attached to its base. The cir- 

 rigerous arms have powerful muscles for their actions, which are of 

 the utmost importance to the animal, inasmuch as the food is obtained 

 by the currents which they produce, and almost incessantly maintain, 

 in the surrounding water. The sessile Barnacles are provided with a 

 series of muscles attached to the margin of the conical shell, which act 

 on the opercular calcareous pieces, and close the opening of the shell. 



The nervous system, as shown in this dissection of the Lepas vitrea 

 by Mr. Goadby, corresponds with that which has been described and 

 figured by Cuvier in the common Lepas anatifera. The oesophagus 

 is surrounded by a wide oval ring, at the sides of which are placed 

 the small ganglions, which supply the first pair of feet. The ring is 

 completed below by the ganglions of the second pair of feet. The 

 fifth and sixth pairs of ganglions are approximated to each other : 

 there is no cerebral ganglion ; but filaments are given off from the 

 supra-oesophageal loop, to the peduncle and sides of the head : two of 

 these branches pass to a small ganglion on either side near the 

 stomach, from which the digestive system is supplied : the tubular ex- 

 tensile tail receives the two last pairs of nerves. The nervous system 

 is perhaps the only part of the organisation of the mature animal which 

 unequivocally indicates the relations of the Cirripedia to the primary 

 divisions of the Animal Kingdom ; it is homogangliate : inferior to that 

 of the Anellides in the low development of the cerebral or supra- 

 cesophageal part, but nearer the crustaceous type in the large size of 

 the ganglions on the abdominal chords. In the degree of approxima- 

 tion of the two chords to each other, the Lepades most resemble the 

 lower isopodous Crustacea, for example, the Talitrus. The neuri- 

 lemma is stained by a dark brown pigment in the Lepas vitrea. 



* Nos. 62. 68, 69. 



