684 



CIRRHOPODA. 



meister also places them amongst the Crus- 

 tacea. De Blainville arranges them, under the 

 name of Nematopoda, as a class of his subtype 

 of the Mollusca Mollusc-articulata ; the 

 other class of the subtype being formed of 

 the Chitons (Polyplakiphora). He regards 

 them as Crustaceous Mollusca, but admits 

 that they seem to form a transition group 

 uniting the Crustacea with the Annelida. M. 

 St. Ange,* however, would rather class them 

 with the Annelida, on account of the closer 

 resemblance which the arrangement of their 

 nervous system bears to that of these animals. 

 Professor Wagner does not doubt that they are 

 really articulated animals, but he would rather 

 place them in a distinct class between the 

 Mollusca and Articulata. Setting aside their 

 nervous system, M. Serres sees, in the other 

 parts of their structure, points enough to in- 

 duce him to arrange them with the Mollusca. 

 The same views are entertained by Wiegmann, 

 Goldfuss, and others. Dr. Leach regarded 

 them as truly annulose animals. Dr. Grant 

 (who calls them " entomoid animals enclosed 

 in shells") places them amongst the Articulata, 

 or diploneurose animals, between Rotifera and 

 Annelida, making of them a distinct class, but 

 admitting their great resemblance in many 

 points to the entomostracous Crustacea. Mr. 

 J. V. Thompson (whose admirable researches 

 on the development of the Cirripeds have 

 thrown a new interest around them) holds it as 

 proved by his observations that the Cirripeds 

 do not constitute a distinct class; but that they 

 are naturally and closely connected, on the one 

 hand, with the Decapod Crustacea, through 

 the Balanids, and, on the other, with the 

 Entomostraca, through the Lepads ; further, 

 that they have no relation with the Testacea. 



All the known Cirripeds may be naturally 

 grouped into two families, one pedunculated, 

 the other sessile. The former includes all the 

 barnacles, properly so called ; the latter, the 

 acorn-shells. The barnacle family have had 

 the name of Campylosomata applied to them 

 by Dr. Leach, who calls the other family 

 Acamtosomata : but we shall use De Blain- 

 ville's synonyms of Lepadicea and Balanidea. 

 The following are the names of the genera 

 generally used at present: 



I. LEPADICEA. 



1. Otion. 2. Cineras. 3. Anatifa. 4. 

 Pollicipes. 5. Scalpellum. 



II. BALANIDEA. 



1. Balanus. 2. Ochthosia. 3. Conia. 



4. Creusia. 5. Clisia. 6. Pyrgoma. 



7. Acasta. 8. Coronula. 9. Tubici- 



nella. 10. Chelonobia. 

 External coverings and organs of support. 

 There are three principal modifications of the 

 tegumentary organs in this class. The first is 

 that seen in Anatifa, in which it assumes the 

 form of calcareous plates, united by horny 

 ligament, and attached to a cartilaginous pe- 

 duncle. The second form is that common to 

 all the Balanids a calcareous cone, composed 

 of separable pieces, sessile, and provided with 



* Mem. sur les Cirripedes. Paris, 1835. 



Fig. 332. 



a 



an opercule of shelly plates. The third form 

 is a general cartilaginous covering, sometimes 

 strengthened by small calcareous plates. 



The shells of the Cirripeds are similar in 

 general appearance to those of many Acepha- 

 lous Mollusca. They are most fully developed 

 in Anatifa, which has five separate plates, four 

 placed laterally in pairs, and one median. 



One pair is conside- 

 rably larger than the 

 other (c, Jig. 332); it 

 covers all the anterior 

 part of the animal, and 

 the greater part of the 

 internal organs. The 

 bases of these shells 

 are attached to the car- 

 tilaginous peduncle ; 

 the lower halves of 

 their anterior edges 

 form part of the mar- 

 gin of the slit-like 

 opening through which 

 the arms are protruded 

 (f,g, .^-332). The 

 inferior pair of shells 

 (d) are of a triangular 

 form ; the smallest side 

 completes the margin 

 of the brachial ori- 

 fice ; another side is united by ligament to the 

 upper valve; the third is connected with its 

 fellow by the common intervalvular ligament. 

 The median piece (e} covers the dorsal aspect 

 of the animal. It has an elongated lanceolate 

 shape, curved and grooved internally. Its 

 upper point only is inserted into the peduncle. 

 Its margins are imbedded in the intervalvular 

 ligament. This piece may be compared to the 

 unpaired valve of the shell of Pholas : it oc- 

 cupies nearly the same situation. The surface 

 of these shells is generally denuded of epi- 

 dermis, excepting just around their margins. 

 All three are strongly and regularly marked 

 with lines of growth, from which it is seen 

 that the two pairs of lateral valves increase in 

 size, chiefly, by additions to their margins, 

 which look towards one another ; so that the 

 parts first formed are, in the adult animal, re- 

 moved to the greatest possible distance from 

 one another. In the upper valve, the umbo or 

 centre of growth is situated in the anterior- 

 superior angle, close to the termination of the 

 peduncle; in the lower, it is situated in the 

 anterior-inferior angle ; and in the dorsal valve, 

 in the point next to the peduncle. All the 

 shells are thin, diaphanous, of nearly the same 

 thickness throughout, yet much less fragile 

 than shells of Acephalous Mollusca which 

 otherwise resemble them. It has been re- 

 marked by Burmeister that the shells of Cir- 

 ripeds resemble those of crustaceous animals 

 more than those of Molluscs : to us it appears 

 that they have a greater degree of density, and 

 a more compact crystalline structure than are 

 commonly met with in Crabs ; and that their 

 well-marked lines of growth give them a closer 

 resemblance to shells of acephalous mollusca. 

 In some genera, as Pollicipes, in addition to 



