380 



R. T. GUNTHEK. 



In Sagitta this primitive independence of right and left 

 coelomic spaces is retained^ but the male and female portions 

 of the hermaphrodite gland have become separated by the 

 transverse genital septum, and a second pair of gonad ducts 

 has come into existence in consequence (test-fig. 10, b). 



See Addendum, page 394. 



Development. 



It now remains to be demonstrated that the developmental 

 histories of the ChEctognatha and Mollusca are not so diverse 

 as has been usually supposed, for many close comparisons 

 are possible between them. 



In the larval stage the Chaetognatha have no velum or 

 other trace of external ciliatiou, and, in this respect, they 

 resemble the Cephalopoda. The late appearance of cilia may 

 be accounted for by the early development taking place inside 

 an egg-shell, and by the larva, on hatching, being already so 

 advanced as to have its adult swimming organs sufficiently 

 developed. The typical Mollusc, on the other hand, hatches 

 at an earlier stage, and goes through a period of swimming 

 by cilia before settling down to the method of locomotion 

 peculiar to its adult condition. So, although we have no 

 evidence that the Chaetognath ancestor was provided with any 

 ciliai'y swimming organ, such a thing is very probable; and 

 Grassi's suggestion that the ciliated olfactory ring is the sur- 

 vival of a larval organ (although he did not indicate the right 

 one) may not be far from the truth. 



We will now pass to the chief features of resemblance in 

 the embryology. The invaginate gastrula is common to both. 

 The mouth has been proved to develop by a stomodaoal 

 invagination at the pole opposite the blastopore both in those 

 Gastropoda which have little food-yolk and in the Cha3to- 

 gnath, and the same statement is true of the Scaphopoda and 

 Lamellibranchia and of Dondersia, though not of Chiton. 



In Mollusca the mesoblast cells usually arrange themselves 

 in two lateral masses, in which paired ccclomic cavities 

 subsequently appear as schizococlic spaces. Erlanger was 



