178 Annelida. 



open into the body-cavity by large funnels,* and to the exterior by delicate 

 apertures at the bases of the limbs. The sexes are separate : the paired gonads 

 open at the posterior end. The species, as a whole, is viviparous. 



Recently Peripatus has been very generally classed with the Arthropoda, 

 chiefly on account of the presence of the air-tubes mentioned above, which are 

 like the tracheae of Insecta, and Myriapoda. But there are weighty facts for 

 the other side : the eyes are of the same kind as those of the Chsetopods, and 

 quite different from the Arthropod type ; a complete set of segmental organs is 

 never found elsewhere in the Arthi-opoda; in the Tracheata, indeed, they are 

 entirely wanting ; also the character of the muscle cells is altogether opposed 

 to a relationship with the Arthropods which exhibit striated muscle fibres. 

 Under the circumstances it seems best to regard the air-tubes as merely 

 analogous with the tracheae, attributing their presence to a ten-estrial life, 

 whilst they are (c/. Insecta) the cause of the degeneration of the vascular system. 



The species of this group live exclusively in wai-m climates in both hemi- 

 spheres (W. Indies, Cape, and elsewhere), in damp places, in rotten wood, etc. 



Appendix to the Annelida. 



Each of tlie groups now to be discussed, the Polyzoa and the 

 Brachiopoda, occupies an isolated position in the Animal 

 Kingdom : it is doubtless therefore most correct to treat them as two 

 special phyla. They were formerly placed with the Mollusca, with 

 which, however, they are not at all closely allied. From the most 

 recent researches, it seems that their nearest relatives — though even 

 these are sufficiently remote — are the Annelids, wherefore they are 

 taken in this connection. 



Polyzoa {Moss-animals). 



With a single exception, all the Polyzoa form colonies by 

 budding ; individual members attain to only a small size, but the 

 extent of the whole colony may be very considerable. The rather 

 short body of each zooid is usually divided into a fore and a 

 hind portion : the latter is covered with a firm, thick, some- 

 times spiny, chitinous investment, the ectocyst, which is often 

 calcified. The front part is, on the other hand, quite soft, and 

 bears at its anterior extremity, a wreath of long ciliated tentacles 

 (the lophophore). In the great majority of forms, this is a simple 

 circle, but sometimes there is a large sinus on one side, which 

 gives it a kidney, or horseshoe, shape. The whole of the front part 

 can be withdrawn into the hinder part by means of a long muscle 

 (Fig. 142) . The wall of the front part is then introverted to form a 

 sheath round the retracted tentacles (tentacle-sheath) . In one 

 section of the marine Polyzoa (the Chilostoma) there is, at the anterior 



* According to some accounts the segmental organ ends in a closed, thin-walled 

 vesicle, not in a funnel. 



