THE CHiETOGNATHA, OR PRIMITIVE MOLLUSOA. 367 



teuthis, Tremoctopus [text-fig. 3], Leioglossa) rather than 

 with the arms themselves. In the Jeep-sea form Sagitta 

 macrocephala it is in a much reduced condition (text-fig. 

 1, c,/.). In development, its bilateral origin already indicated 

 by the notch in the mid-ventral line is very clear, for it is 

 formed from two separate ectodermal thickenings, one on 

 either side of the mouth (Doncaster). 



The two glands near its dorsal attachments may secrete 

 mucus of the inner surface of the hood (text-fig. 1, a, b, gl.). 

 According to the theory that hood and Cephalopod inter- 

 brachial membrane are homologous organs, these hood-glands 

 and the aquiferous pores of Tremoctopus and other Cephalo- 

 poda, cannot be regarded as homologous. 



Musculature and Skeleton. 



The musculature of the Chsetognatha, which has so often 

 been cited as their point of resemblance to the Nematoda, is 

 as distinct as possible from our ordinary conception of 

 Molluscan musculature, but among the varied members of the 

 MoUuscan phylum there are some which present features 

 closely resembling the Chsetognath condition. 



Further, the Chsetognath muscle fibres are striped, whereas 

 Molluscan fibres are as a rule unstriped.^ But were striped 

 muscle entirely unknown in living Mollusca we should not 

 feel inclined to attribute much importance to this difference, 

 believing that muscle striation is merely an indication of 

 efficiency from the point of view of rapidity of contraction. 

 Molluscs are slow in their movements, the ChaBtognatha are 

 quick, that is all. However, Spillmann has shown that well- 

 striated muscle may occasionally appear among the Mollusca, 

 for instance, in the hearts of Turbo rugosus and Acm^ea 

 virginea. 



The arrangement of the longitudinal musculature in a 



1 Several text-books, e. g. those by Lang and Sedgwick, are in error in 

 stating that true striated muscle does not occur in the Molluscan phylum. 



