144 COMPARATIVE ANATOMY. 



§ 114. 



The muscular system of tlie Bryozoa cousists of an external 

 layer of circularj and an internal layer of longitudinal fibres 

 (PhylactolBema). The circular layer is frequently separated into 

 distinct bands. The muscles which connect the protractile portion 

 of the body with the cell are the best developed. When the walls 

 of the cell are very strong the circular bauds are separated (Flustra), 

 and form bundles which pass from the side walls of the cell to its 

 superior free edge. Some of these are inserted into the portion of 

 the cell; which functions as an operculum. When longitudinal 

 muscles are present, some of the muscular fibres are separated off 

 behind the invaginated portion of the body, and pass inwards to the 

 duplicature of the body-wall, whence most of them are produced on 

 to the base of the tentacles. They form the retractors of the 

 anterior part of the body (parieto-vaginal muscles). 



The Vermes differ considerably from one another in the structure 

 of the form-elements of their muscular system. The muscular 

 fibres are more or less elongated structures, which as a rule are the 

 product of a single cell, even where they are very long, as may be 

 inferred from the presence of a single nucleus. The lower forms of 

 the Platyhelminthes have pale fibres often difficult to make out, which 

 may be branched. In the higher Platyhelminthes they form tubes, 

 the contractile substance forming a hollow cylinder, which contains 

 indifferent protoplasm and the nucleus. The contractile portion 

 of the fibres sometimes presents a fibrillar striation. This is seen in 

 the Hirudinea, Acanthocephali, and GejDhyrea. In the last two of 

 these divisions the fibres of each layer form a network. 



Among the Nemathelminthes the simplest condition is seen in 

 Gordius. The muscular fibres are broad thin bands, with their 

 surfaces applied to one another. In others, special differentiations 

 of the fibres may be seen forming rhomboidal plates, which 

 are frequently continued into elongated fibres. The contractile 

 substance is fibrillated and striated, and lies on the outer side of the 

 fibres, while the portion of the fibre directed towards the coelom is 

 formed of protoplasm, which remains indifferent, and encloses a 

 nucleus. With this are allied the special metamorphoses of the 

 fibres into canalicular, . or flattened cylindrical forms. Each fibre 

 has a very deep groove ; this it either retains for' its whole length, 

 or it becomes cylindrical towards its ends ; its open part being always 

 directed towards the body-cavity. The walls consist of contractile 

 substance, broken up into fibrillse. Protoplasm fills the small space 

 of the groove, and a delicate membrane is produced from the edges 

 into a pouch-shaped organ, which projects from each muscular fibre 

 into the body-cavity, the greater part of which is filled up by these 

 pouch-Hke appendages of the muscular fibres (Ascaris lum- 

 bricoides. Fig. 61, A). From the pouches, oblique fibres run to the 

 median lines ; they often have a fibrillar character, and have been 



