4 



BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



Basis 



Basis 



Structure of the individual compartments. — If the basal margin of a compartment, 

 for instance of Balanus tintinnabulum, be examined, it appears sufficiently compli- 

 cated, being composed of an outer and inner 

 lamina, separated by longitudinal septa, 

 which are denticulated at their bases; 

 and the tubes formed by these longitudinal 

 septa are crossed by transverse septa. On 

 the other hand, in some cases, aa in the 

 genera Chthamaltis and Elminius, each, com- 

 partment consists of a simple shelly layer. 

 These two extreme states graduate into 

 each other (in different genera) ; we have, 

 firstly, on the internal surface, qiute irreg- 

 ular points and ridges ; these become regular 

 (in some other genera), causing the interna] 

 surface to be longitudinally ribbed; then 

 these ribs themselves become finely fur- 

 rowed on their sides and at their lower ends, 

 producing sharp, minute ridges, the ends 

 of which I have called the denticuli; and 

 lastly, some of the denticuli on the adjoin- 

 ing longitudinal septa become united into 

 a solid layer, forming the internal lamina 

 of the wall. (Darwin, p. 4'6.) 



Growth of the compartments takes place at the basal margin and 

 usually at the sutures also, where the increment is at right angles 

 to that of the parietes, and fonns the radii. In species which do 

 not increase the diameter of the cone and orifice by the interposi- 

 tion of radii, the orifice is enlarged as the animal grows by wearing 

 away of the summit of the cone. 



Fig. 1.— Wall of a Balanomorph barnacle. 



a, 0, AhM. p, p, PARIETES. T, T, EADn. PART 

 OF THE SUEATH IS VISIBLE IN TUE ORIFICE. IN 



Balanus and many other genera TUE ros- 

 trum AND EOSTROLATERAL COMPARTMENTS 

 ARE CONCRESCENT, NOT SEPARATE AS SHOWN 



HERE (diagrammatic; after Daravtn). 



Fig. 2.— o, Compartment with two radu, serving either as a eostrolateral or as a rostrum 



CONCRESCENT WITH THE ROSTROLATERALS, AS IN BALANUS. 6, LATERAL OR CARINOLATERAL COM- 

 PARTMENT. C, CARINA OR ROSTRUM. LETTERS AS IN FiG. 1 (AFTER DARWIN). 



The upper ends of the pores or tubes in the parietes are more or 

 less extensively filled up with calcareous material, occasionally diifer- 

 ing in color from that of the rest of the wall. In the genus TetracUta, 

 inter alia, the outer lamina of the wall is ordinarily disintegrated 

 and fugacious in adults, leaving a columnar or tessellated surface, 

 fonned of the harder filling of the parietal tubes. 



The lateral edges of the radii are often provided with fine trans- 

 verse ridges or septa, usually having denticulate edges. These fit 



