172 THE ANATOMY OF INVERTEBRATED ANIMALS. 



may be developed in other parts of the body, especially in the 

 region of the head. 



The mouth is usually terminal, but is sometimes ventral 

 and sub-central ; it is ordinarily placed in the centre of a 

 muscular sucker, rarely proboscidiform. The alimentary canal 

 is never provided with an anus. Sometimes a simple sac, it 

 is often bifurcated, and occasionally branched, like that of the 

 dendroccele Turbellaria. Sometimes (Amphilina, Amphipty- 

 ches) the alimentary canal is absent ; and, according to Van 

 Beneden, it becomes aborted in the adult Distoma Jilicolle. 

 The interval between the endoderm and the ectoderm is oc- 

 cupied by a cellular or reticulated mesoderm, in which abun- 

 dant muscular fibres are developed. The peripheral muscular 

 fibres form an external circular and an internal longitudinal 

 layer. 



The water-vascular system is well developed, and may 

 consist of (1) a contractile sac, which opens externally and 

 communicates with (2) longitudinal vessels with contractile 

 non-ciliated walls, from which proceed (3) non-contractile and 

 ciliated branches which ramify through the body, and the 

 ultimate ramifications of which probably end by open mouths, 

 as in the Rotifera. 



There is no pseud-haemal system. The nervous system has 

 not been discovered in all ; but, when it exists, it has the 

 same arrangement as in the aproctous Turbellaria. Eye- 

 spots have been observed, but no other sense-organs. With 

 rare exceptions, the Trematoda are hermaphrodite, and the 

 reproductive organs are constructed upon the same type as 

 in the rhabdoccele Turbellaria, a large vitellarium being al- 

 ways present. The accessory vitellus is included, in the 

 form of numerous pellets, along with the primitive ovum, and 

 is absorbed pari passu with the development of the embryo. 



Aspidocjaster conchic-ola (Fig. 40) inhabits the pericardial 

 cavity of the fresh-water muscle ; it is a very convenient sub- 

 ject for examination on account of its small size, and the ease 

 with which it can be rendered sufficiently transparent for the 

 display of the arrangement of its internal organs, by the 

 judicious use of the compressorium. The flat oval body, 

 rounded posteriorly, is produced in front into a truncated 

 cone, on the face of which the mouth opens. The ventral 

 sucker is very large, and its surface is subdivided into rectan- 

 gular areas. There is no perivisceral cavity, its place being 

 occupied by a mass of spongy cellular tissue. The oral cavity 

 leads into an oval, thick-walled, muscular pharyngeal bulb, 



