688 



CIRKHOPODA. 



the action of which the general mass of the 

 body is drawn deeply within the shell." 

 This we have failed to observe in the species 

 which have come under our notice. When 

 the arms are fully exserted, they are separated 

 one from the other, fan-like. This motion is 

 probably produced by a muscular expansion, 

 described by M. St. Ange as covering the 

 visceral mass dorsally, the fibres of which are 

 grouped into six bundles on either side, cor- 

 responding to the arms. The same observer 

 describes also certain tendons which he found 

 crossing one another at the median line ; these 

 are probably connected with another layer of 

 muscles, expanded over the dorsal surface of 

 the visceral mass, fitted to approximate the 

 arms of either side towards one another. The 

 muscles of the jaws cannot be satisfactorily 

 examined on account of their minuteness. In 

 the Balanids, the valvular opercule is moved 

 by a set of muscles attached to the circle of 

 shelly plates that surround the opening of the 

 parietal cone. Its adductors, which close the 

 aperture with great force, are attached to the 

 extremities of the valves on either side. The 

 visceral mass is, in the Balanids, fixed to the 

 shell by three muscular bands, partly attached, 

 around the mouth, to a process of the epider- 

 mis, and partly spread over the mantle. 



Motility and Sensation. The nervous sys- 

 tem of the Cirripeds consists essentially of two 

 nervous cords running along the abdominal 

 surface, and swelling out into distinctly formed 

 ganglions, at intervals corresponding to the 

 feet-bearing lobes. The first pair of ganglions 

 is situated above the oesophagus (fig- 339). 

 They are united ^,. 



by 1 very short F 'S- 339 ' 



nervous cord. 

 From this supra- 

 cesophageal gan- 

 glion and the u- 

 niting cord, there 

 arise anteriorly 

 three or four 

 nerves, which are 

 distributed to the 

 muscular tunics. 

 The principal ner- 

 vous cords, leav- 

 ing the first gan- 

 glion posteriorly, 

 descend to encir- 

 cle the oesopha- 

 gus. In this 

 course, they give 

 off" branches to 

 the salivary glands and other neighbouring 

 parts, and particularly, (as M. St. Ange has 

 pointed out,) a nerve of communication with 

 a small lateral ganglion (k, k,jig. 339) on either 

 side, situated near the stomach and below the 

 salivary organs. This is connected also with 

 the second pair of ganglions. From this se- 

 cond pair, several branches arise, some of 

 which go to the stomach, and two to the first 

 pair of arms. The other arms receive only 

 one branch eacli (/, /), which is divided into 



cf. 



two, one lor each of the jointed processes. 

 In its course along the abdominal surface, 

 the double ganglionic cord the centre of the 

 nervous system lies immediately beneath the 

 skin, between the bases of the arms. The 

 fifth and the sixth pairs of ganglions have the 

 appearance of being closely united. The tu- 

 bular process, which terminates the anal ex- 

 tremity of the body receives two nerves, one 

 from each of those going to the sixth pair of 

 arms. Dr. Grant directs our attention to the 

 fact that all the anterior parts of this system 

 are very imperfectly developed compared with 

 the posterior parts, and with the same parts 

 in other articulated animals, which have their 

 heads free, and organs of sense more com- 

 plete. 



The sense of touch is the only one enjoyed 

 by the Cirripeds, so far as we can discover. 

 The ciliated arms of some of the species are 

 acutely sensitive : they are withdrawn imme- 

 diately on being touched by any foreign body, 

 and when the surrounding fluid is unfit for 

 respiration. Some observers hare also re- 

 marked that they shrink from a strong light 

 brought to shine upon them suddenly. In 

 the adult animals, there are certainly no 

 organs which can be regarded as eyes; but, 

 according to Mr. Thompson, what he be- 

 lieves to be the free-moving young have very 

 well developed eyes, like those of some crus- 

 tacea. 



Some of the littoral Cirripeds, when left 

 dry at ebb-tide, seem to be sensible of certain 

 changes being produced in the state of the sur- 

 rounding air by the approach of a living being 

 to the place of their habitation. We have 

 frequently remarked, on drawing near a spot 

 densely peopled by the small acorn-shells that 

 so abundantly cover most of our rocks on the 

 sea-shore, a peculiar faint crackling noise, sud- 

 denly produced, gradually subsiding after the 

 lapse of a few seconds, and not repeated 

 until a movement was made towards another 

 spot ; and, on searching for the cause of this 

 singular sound, we have satisfied ourselves 

 that it is uniformly produced by the sudden 

 closing of the opercules of the Balanids, which 

 seem generally to remain open in ordinary cir- 

 cumstances. We have seen this motion again 

 and again follow immediately the movement 

 of the hand towards particular spots, (not, 

 however, nearer the shells than twelve or four- 

 teen inches,) so that we could not but con- 

 clude that the animal was made sensible, 

 through the medium of the air, of the pre- 

 sence of some foreign body, and, fearing dan- 

 ger, closed its shell for self-protection ; just as 

 the limpet, warned of the approach of hurtful 

 agents by the slightest touch of its shell, fixes 

 itself more securely to its rocky footing. 

 What the nature of the sense is which is thus 

 used by the Cirripeds, we have no means of 

 determining. 



Digestion. The minute swimming Crus- 

 tacea appear to constitute the principal food 

 of the Cirripeds. Sometimes, however, the 

 shells of minute Mollusca are found in their 



