CIRRIIOPODA. 



685 



the five valves just described, there are other 

 eight smaller calcareous plates arranged around 

 the junction of the peduncle with the shells. 



The shells of the Balanids present several 

 striking peculiarities of structure, and, in their 

 mode of growth, offer to the physiologist an 

 interesting subject for investigation. They 

 form truncated cones, the bases of which, 

 without the intervention of peduncles, are fixed 

 to rocks, floating wood, integuments of marine 

 animals, &c. These cones are composed of 

 several pieces, closely cemented together so as 

 to admit of no motion between them, excepting 

 during the process of enlargement of the shell. 

 In the common acorn-shells (Jig. 333), which 



cover our litto- 



Fig. 333. ral rocks and 



the bottoms of 

 ships, there are 

 seven of these 

 pieces, six form- 

 ing the walls, 

 and one dis- 

 coid, forming 

 the base. The 

 outer surface of 

 the parietal 

 valves is mark- 

 ed by the lines 

 of growth in 

 such a manner 



as to give it the appearance of being com- 

 posed of twelve pieces. These may be termed 

 compartments. They are all conical. Six of 

 them have their bases applied to the common 

 base of the shell, and the other six are inserted 

 between these, with their apices towards the 

 common base. The first six we shall refer to 

 under the name of thejirst series of compart- 

 ments (a, a, fig. 333) ; the other six constitute 

 the second series (6, b, Jig. 333). The opening 

 in the summit of the cone is closed by an 

 opercule composed of four shelly pieces so 

 arranged as to leave a longitudinal fissure be- 

 tween them, through which the arms are pro- 

 truded (c, Jig. 333). The two series of com- 

 partments differ much from one another in 

 their external aspect, owing to the differences 

 in the directions and appearances of the lines 

 of growth. The second series have a smoother 

 surface, and are marked with very delicate 

 lines, both longitudinal and transverse ; they 

 are also less prominent than the first series. 

 The lines on the first series are chiefly trans- 

 verse, and correspond with the outline of the 

 base. On the internal surface of the walls 

 there are six deep grooves, in the bottoms of 

 which are seen the openings into certain cham- 

 bers, constituting a sort of diploe of the valves, 

 hereafter to be described. These grooves run 

 from the summit to the base of the shell, and 

 are the internal edges of the sutures of the six 

 parietal valves. Around the internal margin 

 of the common base there is a series of holes 

 opening into certain tubes that terminate on 

 the outer margin of the shell. When all the 

 valves are separated at the sutures, it is found 

 that each of four of the six compartments 

 of the first series, as they appear externally, 



has attached to its dorsal margin one of the 

 second series, arid that the union between these 

 two is exceedingly intimate, in fact that they 

 form one piece, notwithstanding their apparent 

 division externally. Two of the second series 

 of compartments are attached to the anterior 

 valve, while the dorsal valve has none. The 

 anteal margins of the lateral valves and both 

 margins of the dorsal valve are marked by 

 transverse depressions corresponding to the 

 numerous partitions of the chambered com- 

 partments which are fitted into them ; and, 

 externally, each has a projecting margin. To 

 the upper part of the inner surface of each valve 

 there is attached a laminated process, form- 

 ing part of a circle of calcareous plates which 

 gives support to some parts of the mantle. 



The internal structure of these shells pre- 

 sents some peculiar features. They all contain 

 numerous tubes and cavities, regularly ar- 

 ranged, and forming a sort of diploe. The 

 suture-holes mentioned above open each into 

 a separate canal, chamber, or tube. Those 

 which occur in rows on the walls of the cone 

 lead to small chambers within the second series 

 of compartments, running parallel with the 

 general base, and separated from one another 

 by delicately-formed partitions, each of whicli 

 is deeply grooved on both sides. The par- 

 titions are placed at equal distances, and their 

 grooves are most regularly formed. The whole 

 presents one of the most beautiful and delicate 

 pieces of structure with which we are ac- 

 quainted in the whole range of extravascular 

 skeletons. These are from thirteen to fifteen 

 on either side of each partition. Fig. 334 repre- 

 sents a perpendicular section 

 of a few of these grooved par- 

 titions considerably magnified. 

 Fig. 335 represents a horizontal 

 section of one of the six valves. 

 The holes forming the sutures 

 are at a. The grooved floor 

 of one of the chambers of the 

 piece is be- 



Fig. 335. tween a, d, 



and c. d, c 

 is the outer 

 wall of the 

 compartment 

 of the se- 

 cond series, a, b is a section of that part of 

 the valve which appears outside as a compart- 

 ment of the first series. Its diploe' is com- 

 posed of tubes, running from the apex to the 

 base, gradually enlarging below. Horizontal 

 sections of those tubes shew them to be of an 

 ovate form, tapering inwardly (Jig. 336). They 

 are placed nearer 



Fig. 336. the outer wall than 



the inner. The 

 spaces intervening 

 between the taper- 

 ing sides of the 

 tubes are marked with lines of growth, shew- 

 ing a gradual filling up of the tubes from 

 within outwards; and also the previous ex- 

 istence of furrows or grooves on the surfaces 

 of the partitions between the tubes. These 



Fig. 334. 



