1101 



CIR1UPEDIA. 



CIRRIPEDIA. 



1102 



from a single specimen described and figured by Burmeister (' Bei- 

 triige zur Naturgeschichte der Rankenfiisser,' s. 16). In its general 

 shape and compressed form it seems to come nearer the last than 

 the first stage. It has only three pairs of legs, situated much more 

 posteriorly on the body than in the first stage, and all directed 

 posteriorly. They are much shorter than in their earlier stages. 

 They are xmdoubtedly the three pairs of limbs of the first stage 

 metamorphosed. The chief development of the larva since its first 

 stage is towards its anterior end. 



In the last stage the larva; have increased many times in size since 

 their exclusion from the egg. They are now much compressed, nearly 

 of the shape of a Cypris, or mussel-shell, with the anterior end the 

 thickest, the sternal surface nearly or quite straight, and the dorsal 

 arched. Almost the whole of what is externally visible consists of the 

 carapace, the thorax and limbs being hidden and inclosed by its 

 backward prolongation, and even at the anterior end of the animal 

 the narrow sternal surface can be drawn up, so as to be likewise 

 inclosed. The antenna) are large and conspicuous. They are at first 

 well-furnished with muscles, and serve as organs of locomotion, and 

 apparently as feelers ; but their main function is to attach the larva, 

 preparatory to its final metamorphosis into a Cirripede. The disc can 

 adhere even to so smooth a surface as a glass tumbler. The attach- 

 ment is at first manifestly voluntary, but soon becomes involuntary 

 and permanent. Mr. Darwin makes the following remarks upon the 

 eyes and mouth in their last stage : 



" Eyes. The posterior and rounded margins of the basal articu- 

 lation of the above-described prehensile antennae are reflected inwards, 

 in the form of two forked horny apodemes, together resembling two 

 letters U U close together. These project up inside the animal for at 

 least one-third of its thickness, from the sternal to the dorsal surface. 

 The two great almost spherical eyes in Lepas australis,ea,ck r^th of an inch 

 in diameter, are attached to the outer arms, thus, U U , in the posi- 

 tion of the two full stops. Hence the eyes are included within the cara- 

 pace. Each eye consists of eight or ten lenses, varying in diameter in 

 the same individual from s^th to s^th of an inch, inclosed in a common 

 membranous bag or cornea, and thus attached to the outer apodemes. 

 The lenses are surrounded half-way up by a layer of dark pigment 

 cells. The nerve does not enter the bluntly-pointed basal end of the 

 common eye, but on one side of the apodeme. The structure here 

 described is exactly that found, according to Milne-Edwards, in cer- 

 tain Crustacea. In specimens just attached, in which no absorption 

 has taken place, two long muscles with transverse stria; may be found 

 attached to the knobbed tips of the two middle arms of the two U U * 

 and running up to the antero-dorsal surface of the carapace, where 

 they are attached. Other muscles (without transverse striae) are 

 attached round the bases on both sides of both forks. The action of 

 these muscles would inevitably move the eyes, but I suspect that their 

 function may be to draw up the narrow deeply-folded sternal surface, 

 and thus cause the retraction of the great prehensile antenna; within 

 the carapace. 



" Mouth. This is seated in exactly the same position as in the 

 mature Cirripede, on a prominence fronting the thoracic limbs, and 

 so far within the carapace that it was obviously quite unfitted for 

 the seizure of prey ; and it was equally obvious that the limbs were 

 natatory, and incapable of carrying food to the mouth. This enigma 

 was at once explained by an examination of the mouth, which was 

 found to be in a rudimentary condition, and absolutely closed, so that 

 there would be no use in prey being seized. Underneath this slightly- 

 prominent and closed mouth I found all the masticatory organs of a 

 Cirrjpede in an immature condition. The state of the mouth will be 

 at once understood if we suppose very fluid matter to be poured over 

 the protuberant mouth of a Cirripede, so as to run a little way down 

 in the shape of internal crests, between the different parts, and in the 

 shape of a short, shrivelled, certainly closed tube, a little way ('008 of 

 an inch in L. auatralii) down the oesophagus. Hence the larva, in this, 

 its last stage, cannot eat. It may be called a locomotive pupa ; its 

 whole organisation is apparently adapted for the one great end of 

 finding a proper site for its attachment and final metamorphosis." In 

 this stage the thorax is much compressed, the six pairs of legs are all 

 close one behind the other. In all the limbs the obliquely-truncated 

 summit of the terminal segment of the inner ramus bears three very 

 long beautifully plumose spines ; in the first pair the summit of the 

 outer ramus bears four, and in the five succeeding pairs six similar 

 spines. The abdomen is small, and consists of only three segments : 

 it contains only the rectum, and two delicate muscles running into 

 two appendages, between the bases of which the anus is situated. 



Whilst the young Lepas is closely packed within the larva, the 

 capitulum, or shell, about equals the length of the peduncle. Even at 

 this early period the muscles of the peduncle are distinct. The com- 

 pound eyea, which we have seen are attached to apodemes springing 

 from the sternal surface of the larval carapace, are consequently cast 

 off' with it. Whilst the young Cirripede is packed within the larva, 

 the outer integument of its peduncle necessarily forms a deep trans- 

 verse fold passing over the eyes and apodemes. This determines the 

 position and origin of the sac in the young Cirripede. 



" The larva," says Mr. Darwin, " fixes itself with its sternal surface 

 parallel and close to the surface of attachment, and the antennae 

 become cemented to it : if the Cirripede after its metamorphosis had 



remained in this position, the cirri could not have been exserted, or 

 only against the surface of attachment ; but there is a special pro- 

 vision that the young Cirripede shall assume its proper position at 

 right angles to the position which it held whilst within the larva, 

 namely, with its posterior end upwards. This is eifected in a singular 

 manner by the exuviation of the great compound eyes, which we have 

 seen are fastened to the outer arms of the double *UU* Mko 

 sternal apodemes. These, together with the eyes, stretch transversely 

 across and internally far up into the body of the larva ; and as the 

 whole has to be rejected or moulted, the membrane of the peduncle 

 of the young Cirripede has necessarily to be formed with a wide 

 and deep inward fold extending transversely across it ; this, when 

 stretched open after the exuviation of the larval carapace and 

 apodemes, necessarily causes the sternal side of the peduncle to be 

 longer than the dorsal, and as a consequence gives to the young 

 Cirripede its normal position, at right angles to that of the larva when 

 first attached." 



That the homologies of the larva of the Cirripedes are with 

 the Crustacea has before been pointed out. Mr. Darwin says : " In 

 Crustacea, according to the ordinary view, there are twenty-one seg- 

 ments ; of these I can recognise in the Cirripede, on evidence as good 

 as can generally be obtained, all with the exception of the four 

 terminal abdominal segments ; these do not occur in any species 

 known to me in any stage of its development. If that part of the larva, 

 in front of the mouth, bearing the eyes, the prehensile antennae, and 

 in an earlier stage another pair of antennae, be formed, as is admitted 

 in all other Crustacea, of three segments, then beyond a doubt, from 

 the absolute correspondence of every part, and even every coloured 

 mark, the peduncle of the Lepadidce is likewise thus formed. The 

 peduncle being filled by the branching ovarian tubes is no objection 

 to this view, for I am informed, on the high authority of Mr. J. D. 

 Dana, that this is the case with the cephalo-thorax in some true 

 Crustaceans ; for instance, in Sapphinna. To proceed : the mouth, 

 formed of mandibles, maxilla;, and outer maxilla;, corresponds with 

 the fourth, fifth, and sixth segments of the archetype Crustacean. 

 Posteriorly to the mouth we come, in the larva, to a rather wide 

 interspace, without any apparent articulation or organ ; and then to 

 the thorax, formed of six segments, bearing the six pairs of limbs, of 

 which the first pair differs slightly from the others. The thorax is 

 succeeded by three small segments differently shaped, with the 

 posterior one alone bearing appendages. These segments I cannot 

 doubt, from their appearance alone, and from their apparent function 

 of steering the body, are abdominal segments. If this latter view be 

 correct, the thoracic segments are the six posterior ones of the normal 

 seven segments ; and there must be two segments missing between the 

 outer maxillae and the first thoracic pair of legs, which latter, on this 

 view, springs from the ninth segment. Now, in a very singular 

 Cirripede named Proteolepas, the two missing segments are present, 

 the mouth being actually succeeded by eight segments, and these 

 by the three usual abdominal segments, every segment in the body 

 being as distinct as in an Annelide ; hence in Proteolepas, adding the 

 three segments for the mouth and three for the carapace, we have 

 altogether seventeen segments, which, as I have stated, is the full 



Fig. 1. A Stomapod Crustacean (Lmcifer, V.Thompson. 

 Fig. 2. A species of Lepas. 



number ever observed in any Cirripede ; the four missing ones being 

 abdominal, and I presume the four terminal segments. That the 

 cavity in which the thorax is lodged in the larva, and therefore in 

 the mature Cirripede, is simply formed by the backward production 

 of the carapace, does not require any discussion. The valves have 

 no homological signification." 



The preceding wood-cut, copied from Darwin's work, will make 

 these homologies clear. The upper figure is a Stomapod Crus- 

 tacean (Leucifer, V. Thompson), and the abdomen, which becomes 

 rudimentary in Cirripedes, is given in faint lines. The lower figure 

 U a mature Lepas with the antenna; and eyes which are actually 

 present in the larva, retained and supposed to have gone on growing. 

 All that is seen of a Cirripede, whether pedunculated or sessile, is 

 the three anterior segments of the head of a Crustacean, with its 

 anterior end permanently cemented to a surface of attachment, and 

 with its posterior end projecting vertically from it. 



