u6 THE CRUSTACEA 



it communicates, in the Pedunculata, with a canal traversing 

 the length of the peduncle. 



Excretory System. The excretory organs of the Cirripedia have 

 been the subject of much discussion, and it is only recently that 

 their structure has been clearly ascertained. Darwin described as 

 " olfactory " organs a pair of minute orifices, sometimes elevated on 

 tubular papillae, on the outer side of the maxillae. These were 

 shown by Hoek to be the openings of a pair of fine canals, which 

 he regarded as " segmental organs " and described as opening into 

 the body-cavity. This cavity has been shown by Bruntz to be the 

 greatly enlarged " labyrinth " of the maxillary gland, the end-sac of 

 which, discovered by Nussbaum, communicates with the labyrinth 

 by a minute aperture. The end-sac is of considerable size, and 

 may be divided by internal partitions. In addition to these 

 maxillary glands, an excretory function is discharged by the hepatic 

 caeca and by scattered " nephrocytes," the most important of which 

 are aggregated in two masses at the sides of the cephalic region. 



Glands. A very peculiar and characteristic feature in the 

 organisation of the normal Cirripedia is the cement apparatus by 

 means of which the attachment of the animals is effected. This 

 consists of a pair of much-ramified follicular glands connected with 

 a pair of ducts which open, in the later larval stages at least, on 

 the antennules. In the Pedunculata these glands are lodged in 

 the peduncle (Fig. 67, Cd), and in the Operculata in the thickness 

 of the basal plate (Fig. 64, gl.ce). In many Pedunculata (Lepas, 

 Conchoderma, etc.) the openings of the ducts on the antennules 

 serve throughout life for the escape of the cement, but in others 

 (Scalpellum, Pollicipes) numerous additional apertures are formed 

 on the surface of attachment. In the Operculata the glands and 

 their ducts ramify in a complex way over the broad base and 

 discharge the secretion through numerous pores. In one species 

 of Lepas (L. fascicularis) the cement forms, at the end of the 

 peduncle, a vesicular mass, serving to increase the buoyancy of the 

 floating colony. 



In the Acrothoracica, which bore into the shells of Mollusca 

 and into corals, the cement-glands are much reduced and are prob- 

 ably functional only in early life. 



Muscular System. In the great majority of the Pedunculata 

 and in the Operculata a strong adductor muscle (Fig. 64, m.a.s, 

 Fig. 67, Tlf) connects the two scuta dorsal to and in front (on the 

 rostral side) of the alimentary canal. When the scuta are absent 

 the muscle is attached on each side to the cuticle of the mantle. 

 In the genus Ibla an adductor scutorum is also present, but as it 

 crosses the body on the ventral side of the alimentary canal 

 between the oesophagus and the ventral nerve-mass it cannot be 

 regarded as homologous with the similarly named muscle of the 



