ACANTHOCEPHALA : BRACHIOPODA, &c. 691 



Saefftigen, M. J. x. 1884 ; Baltzer, A. N. 46, 1880 (contains litt.). Development, 

 Leuckart, Die Menschlichen Parasiten, ii. Leipzig, 1876. Female sexual organs of 

 Ech. gtgas, Andres, M. J. iv. 1878. Larva of Ech. clavaeceps, Villot, Z. A. viii. 1885. 



BRACHIOPODA, VERMIFORMIA, POLYZOA, PTEROBRANCHIA. 



The systematic position and relations of these four classes still 

 remains, in spite of much research, a matter of complete uncertainty. 

 They are often grouped together as Molhiscoidea. The term is not used 

 here because there does not appear to be any reason to approximate them 

 to the Mollusca. If, however, recent views as to the resemblance between 

 the larval Loxosoma and the Molluscan Trochosphere prove correct, the 

 entoproctous Polyzoa will have to be separated in all probability from the 

 other Polyzoa, as well as from the remaining classes, with the possible 

 exception of Pterobranchia. 



The nearly complete atrophy or the total absence of a praeoral region, 

 the presence of a lophophore bearing ciliated tentacles, of a lip or epi- 

 stome overhanging the mouth, and of a fixed mode of life are characteristic 

 of this assemblage of forms. The Brachiopoda and Vermiformia are social, 

 the Polyzoa and Pterobranchia form colonies by gemmation. The two 

 first possess a coelome, which is an enterocoele, and at least one pair of 

 nephridia, the Pterobranchia and ectoproctous Polyzoa a coelome, the 

 entoproctous nephridia but no coelome. As to the lophophore it may be 

 remarked that in Brachiopoda, Vermiformia, and apparently in ectoproctous 

 Polyzoa, it is postoral, in entoproctous Polyzoa praeoral, in the Ptero- 

 branchia doubtful in position on account of the development being as yet 

 completely unknown. The epistome of the entoproctous Polyzoa appears 

 to be homologous with the foot of the Molluscan Trochosphere, and in 

 Pterobranchia it is not only an organ of locomotion but also secretes, at 

 least in Rhabdopleura, the tube in which the animal lives. In BracJiiopoda 

 and Vermiformia, certainly in the last, it is a remnant of an atrophied 

 praeoral lobe. The position of the mouth within the lophophoral area, as 

 in Brachiopoda and Vermiformia, would tend to the supposition that the 

 epistome of the phylactolaematous Polyzoa is also a remnant of the same 

 region. It is however often regarded as homologous with the foot of 

 Pterobranchia. The peduncle of the Vermiformia is a ventral growth ; 

 there is no reason to suppose that the corresponding structure in Brachio- 

 poda is anything of the kind ; nor are the divisions of the coelome homo- 

 logous in the two classes as has been supposed ; nor again is the anus of 

 Brachiopoda to be considered as dorsal in position as it is in Vermiformia. 



There is some slight evidence in favour of considering Brachiopoda as 

 primitively segmented animals, not on account of the segmentation of the 



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