522 WILLIAM BATESON. 
The Tail and Anal Lappets. 
The tail is present in the period between one and eight pairs 
of gill-slits. Its skin is full of unicellular glands. The third 
pair of body cavities are prolonged into it, and the mesentery 
between them remains. The anal lappets (fig. 3, a) also dis- 
appear with the tail. 
Mesoblastic Structures. 
Muscles.—The muscle-fibres of the proboscis are not 
gathered into bundles. They consist of circular, radial, and 
longitudinal fibres. The circular fibres are few in number, and 
chiefly occur in the external parts of the middle third of the 
proboscis. 
The radial fibres are very few in B. Kowalevskii, but in 
B. salmoneus and B. Robinii they are common, and have 
a very characteristic appearance (v. fig. 94, a). Their peri- 
pheral ends are very long and fine, occasionally branching. 
Their central ends taper suddenly from a thick part containing 
a nucleus to a very fine fibre. These fibres are always plain 
fibres. Probably the peripheral ends are inserted into the 
skin, and the central end into the meshes of connective tissue 
which permeate the body cavity (v. fig. 79). 
The longitudinal fibres of B. Kowalevskii are arranged in 
concentric rings, and united to each other by a peculiar con- 
nective tissue, which contains stellate cells with large nuclei. 
These concentric rings seem to be more numerous in old than 
in young animals, reaching the observed maximum of eight. 
This concentric arrangement is not a distinct feature until 
adult life is nearly reached. ‘These fibres appear in section to 
have the same structure as those shown in fig. 94, 4, which is 
taken from B. Robinii. The muscles of B. Kowalevskii 
were unfortunately not examined in the fresh state. 
In B. minutus the longitudinal muscles do not form such 
definite concentric rings as in B. Kowalevskii, but all the 
mesoblastic tissues filling the proboscis cavity are broken up in 
preserved specimens into radial segments. This is not the case 
