The results obtained in this paper may be summarized as follows :

1. The body-wall in the majority of the Cyclostomata consists of cuticle, calcareous layer, ectoderm, and mesoderm. The two former layers are secreted from the ectoderm. The cuticle is found only outside the calcareous layer. The ectoderm and mesoderm are both strongly reduced. The so-called pores that pierce the calcareous layer are not real pores but pseudopores ; the interzoidal pores are, on the contrary, always real pores. There is never any cellular layer outside the cuticle of the bodywall.

2. In the joints that occur in the Crisiidae there is, instead of the calcareous layer, a firm chitinous substance forming an annular zone that is wider on the inward side than on the outside of the joint. The cuticle is torn off all round the joint. The ectoderm and mesoderm are much better developed in the joint than in the rest of the body-wall.

3. In the Horneridae and the Lichenoporidae the body-wall is double, consisting of a gymnocyst and a cryptocyst, separated by a slit-like hypostegal coelomic cavity. The gymnocyst is composed of cuticle, ectoderm, and mesoderm, whereas the cryptocyst consists of a calcareous layer on both sides surrounded by ectoderm and mesoderm. The interzoidal walls, in the two families mentioned, are of the same structure as in all other Cyclostomes.

4. The cryptocyst in the Horneridae and the Lichenoporidae is not, in all probability, homologous with the formation designated by the same name and occurring in many Cheilostomes.

5. On account of the structure of the body-wall there are no pseudopores in the Horneridae and the Lichenoporidae, but only real pores. In the Lichenoporidae there are no pores at all in the walls of the autozoids.

6. It seems very probable that the structure of the body-wall in the family Heteroporidae and in the extinct sub-order Trepostomata is the same as in the Horneridae and the Lichenoporidae.

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