44 BALANID.E. 



Tubes are formed by the longitudinal septa, between the 

 outer and inner laminae. These tubes are almost square, and 

 are occupied by threads of corium, which enter at pores 

 left open between the edge of the compartment and that of 

 the basis on which it rests. The tubes extend high up the 

 compartments; but in the uppermost part they are generally 

 cut off by thiii; transverse, calcareous septa, deposited by 

 the ends of the threads of corium ; a cancellated structure 

 being thus produced. Or the uppermost part of each tube 

 becomes filled up solidly with compact shelly layers, which 

 are always first thrown down on the side of the tube facing 

 the outside, and thus greatly strengthen the shell : in 

 several instances, as in Balanus perforatum and Tetraclita 

 porosa, in which the disintegration of the upper part of the 

 shell is a necessary element in its growth for the enlarge- 

 ment of the orifice, these filled up tubes become exposed. 

 In Coronula and Tubicinella, the tubes in their upper 

 ji arts are, I believe, crossed only by transverse membranous 

 septa. 



Anomalies and exceptions. — In Tetraclita (PL 10, fig. ly, 

 1 //) from the branching of the longitudinal septa, several irre- 

 gular rows of tubes are formed. In certain varieties of 

 Balanus balanoides (PI. 7, fig. 2b), and in B. cariosus 

 (PI. 7, fig. 3 5), slight branching ridges on the internal 

 surface of the walls, seem to answer to the longitudinal 

 septa, and produce, during the downward growth of the 

 shell, extremely irregular cells, and short tubes. In Balanus 

 vinaceus (PI. 2, fig. 7 d), the internal lamina, instead of 

 being solid, as in every other species, is left cancellated, and 

 thus betrays, much more plainly than usual, its origin in 

 the united denticuli of the adjoining longitudinal septa. In 

 Balanus porcatus, between the main longitudinal septa, 

 there are (PI. 0, fig. 4 e) what may be called rudimentary 

 and disconnected longitudinal septa. In Coronula and its 

 allies (PL 16, fig. 6, and PL 17, fig. 4 c) it is the outer 

 lamina of the compartment which is anomalous ; for in the 

 two or three lower zones of growth, it forms only a 

 ledge on each side of the longitudinal septa j which ledges, 

 higher up, become confluent, and so form an ordinary outer 

 lamina. In Coronula, also, the wall of each compartment 



