236 LOWER INVERTEBRATES. 



Branch YI. — MOLLUSCOIDEA. 



Among the forms of disputed position occur two well-marked groups of aquatic 

 animals, the Brachiopoda and the Polyzoa. In the older works the former of these 

 was included among the Mollusca, while the second was accorded a place along with 

 the hydroids in the heterogeneous group of Zoophytes. The next step was to recog- 

 nize the affinities of the two, and, as a consequence, the Polyzoa were placed alongside 

 the Brachiopoda, as members of the great group Mollusca. Then embryology was 

 invoked, and, led by certain resemblances, many naturalists separated the brachiopods 

 from the molluscs and gave them a place among the worms, the Polyzoa being dragged 

 along with them. At present the tendency seems to be to recognize the aflBnity of 

 the two groups to both the worms and the molluscs, while assigning them a place 

 intermediate between the two. 



At first sight the Polyzoa and the Brachiopoda seem widely different, but a deeper 

 knowledge reveals many and important points of contact, especially in their early 

 stages. They are all, with few exceptions, attached to some sub-aquatic object. In 

 the adult all traces of metameric segmentation are lost. The tentacular apparatus 

 is ciliated, and is borne upon a circular disc, or a two-armed process arising from the 

 oral region. The alimentary canal forms a single loop, the mouth and anus (when 

 the latter is present) being near each other. The principal nervous centre is a 

 ganglion just beneath the oesophagus. 



The larvfe of each, though presenting themselves in various forms, can be reduced 

 to a common type, consisting of a body divided into two regions by a ring of cilia, 

 in the anterior of which is the mouth, and not infrequently the anus as well. This 

 larva can be but little removed from the trochozoon, the hypothetical ancestor of the 

 worms and molluscs. Some recent investigations tend to show that the relationships 

 supposed to exist between the two groups are really those of analogy, and not of 

 homology, and that the Polyzoa have some connection with the rotifers, a group here 

 treated of among the worms. 



Class I. — POLYZOA. 



The term Polyzoa, which means many animals, is highly appropriate for the group 

 of aquatic forms which we now take up, for they, with a very few exceptions, form 

 colonies composed of many individuals united in a common stock. Another name, 

 which was applied but a year later, is Bryozoa, or moss-animals, a term no less apt, 

 but debarred from use by the law of priority which governs scientific nomenclature. 

 In general appearance many of the group closely resemble the hydroids, and especi- 

 ally the sertularians. Like them, they have a compound structure, and are enveloped 

 by a cuticular sheath, while the circle of tentacles, which in life projects from the 

 openings of the little cups, still furthei- strengthens the similarity, and affords a justi- 

 fication for their association with those lower forms by the older naturalists. But 

 when we come to study the anatomy and the development of these animals, important 

 differences at once show themselves, and the resemblances which at fii'st struck us so 

 forcibly are seen to be of a merely superficial character. 



