686 



CIERHOPODA. 



Fig. 338. 



grooves are very strongly marked in some spe- 

 cies, as in Balanus Spinosus (Jig- 337), where 



the tubes are large, and 

 Fig. 337. the walls comparatively 

 thin. In all they run in 

 straight diverging lines 

 from the apices of the 

 compartments to their bases. There they open 

 close to the margin of the general base. In 

 most species, however, their orifices are, in 

 part, filled up by an extension of the base (, 

 Jig. 338). In some small species, the tubes of 

 which are wider than those of larger ones, 

 there is hardly any opening- 

 discoverable externally, or at 

 most a very narrow fissure just 

 around the margin. Very near 

 their terminations on the mar- 

 gin, these tubes of the diploe 

 are joined by the very short 

 canals which proceed from 

 the inner circumference of the 

 base (6, Jig. 338), and it is at 

 their junction that the grooves 

 in the walls of the partitions are most obvious. 

 These two sets of tubes communicate freely all 

 around the margin with the diploe of the base. 

 All the Balanids with the exception of the 

 Coronules have calcareous bases. The struc- 

 ture of the base differs from that of the walls 

 in being composed internally of large oval 

 cells irregularly arranged. These cells seem 

 to communicate freely with one another and 

 with the tubes of the valves. The Coronules 

 have no base : their soft parts are in immediate 

 contact with the integuments of the living 

 animals in which they are generally imbedded. 

 The form and arrangement of the opercule 

 vary. There are generally four triangular 

 valves, two larger than the others, all deeply 

 grooved on their upper surfaces by the lines 

 of growth. These valves cover more or less 

 completely the soft parts beneath, to which 

 they are attached, so as to be very moveable 

 one upon the other, and to admit of the pas- 

 sage of the feet through the slit that exists be- 

 tween the two pairs. In some of the Coronules, 

 the greater part of the opercule is soft. Coro- 

 nula diadema has two small shelly plates in its 

 opercule. 



Keeping in view the complex but beautiful 

 structure just described, it is not difficult to 

 determine how the whole shell increases in 

 size. It is obvious that the parietal compart- 

 ments of the first series are enlarged by addi- 

 tions to their basilar edges and internal surface, 

 and that thus the whole cone is lengthened, 

 and consequently widened at its base; but, 

 in all the species, it is also widened above ; 

 and, as the summits of the first series of com- 

 partments are, evidently, not at all, or, at most, 

 very slightly, abraded by the friction of the 

 opercule, it is certain that the apices of these 

 compartments originally very closely approx- 

 imated must be moved outwards and sepa- 

 rated from one another by the gradual increase 

 m breadth of the intervening wedge-like com- 

 partments of the second series. This process 

 jnnpljcs the insertion of soft parts endowed 



with vascular action between the valves so as 

 to admit of lateral additions being made to 

 the second set of compartments. There can 

 be no question that these soft parts (foliated 

 processes of the mantle) pass into the sutures 

 along their whole length, and deposit the 

 shelly matter on the edges of the partitions 

 forming the chambered structure of the se- 

 cond series of compartments ; each valve, with 

 the exception of the dorsal one, is thus added 

 to in breadth ; and as the distance between 

 the original valves is enlarged, and the whole 

 shell lengthened, new chambers are formed 

 below. Of course, as the cone is lengthened, 

 its base is widened ; and this is effected by 

 the excretion of shelly matter from such parts 

 of the mantle as can easily pass through the 

 numerous holes placed around the inner cir- 

 cumference of the base. The valves of the 

 opercule are imbedded in the margins of the 

 mantle between the epidermis and true skin, 

 and are increased by marginal additions in the 

 same way as the shells of molluscs. 



The mode of growth of these shells engaged 

 the attention of Cuvier, who concluded that an 

 addition to the sides of the valves could take 

 place only in an early age ; for it appeared to 

 him that they are, in a more advanced stage, 

 so firmly cemented together as not to admit 

 of separation. In large species, however, we 

 find that the valves are easily separated at the 

 sutures, and that the calcareous matter along 

 the sides of the sutures is loosely aggregated ; 

 so that, to us, there seems to be no impro- 

 bability in the supposition that in the living 

 animal the prolongations of the mantle pass 

 between the terminations of the minute tubu- 

 lar processes of the second series of compart- 

 ments, and the corresponding depressions in 

 the edges of the first series already noticed. 

 There is no indication, we think, of each of 

 the valves being " detached from its neighbour 

 only at certain times that it may receive addi- 

 tional calcareous matter along its sides," as 

 Brugieres and Cuvier imagined. The process 

 of growth seems to be carried on in uniform 

 progression until adult age. So puzzling did 

 the problem of the mode of growth in these 

 shells appear to Dufresne, that he concluded 

 that, like crabs, the Balanid casts its old shell, 

 and forms a new one, as it increases in size.* 

 Cuvier remarked that, " while the mode of 

 growth of the shells of the Mollusca resembles 

 that of simple teeth, the organization and in- 

 crease of the shells of balanids may be com- 

 pared to that of certain compound teeth, par- 

 ticularly those of diodons and tetrodons." 



Tubicinella, a parasite of the Whale, differs 

 much from the other balanids in the formation 

 of its shell. The widest part of its six-valved 

 cone is superior; the whole surface is strongly 

 ribbed, and marked with transverse lines of 

 growth ; and it appears that the additions to 

 the cone are made on the upper margin ; this 

 margin is surrounded internally by a thick and 

 fleshy production of the mantle, which is never 

 altogether covered by the opercule. The base 



* Ann. du Mus. i. 467. 



