92 Evolution and Distribution of Fishes 



Each sac, as described and figured by Dendy, is a swollen 

 expanse of mesoblastic tissue lined by epithelial cells, and 

 that opens by a duct externally along the dorso-lateral 

 aspect. In cyclostomes the paired relation has been lost, 

 though it is retained in other fishes. But the sacs are also 

 mesoblastic and similarly constructed. The discharge of 

 the sperms or eggs, however, is effected in one of three 

 different ways, that are doubtless graded derivations from 

 the primitive nemertean type. The simplest is shown by 

 cyclostome fishes, where, instead of the sacs opening indi- 

 vidually by external pores as in nemerteans, the ducts of 

 the former now open into the body-cavity, and from here 

 the products are set free through two (Petromyzon) or one 

 pore, that opens into a cloacal cavity, and thence to the 

 exterior. In other fishes more complicated conditions have 

 developed, that fall under one of two heads. Thus, in 

 elasmobranchs, holocephalans, and possibly also in Dipnoi, 

 a split-off branch of the segmental or archinephric duct be- 

 comes the MuUerian duct that opens into the body-cavity, 

 receives the reproductive products, and conveys these 

 through the cloaca to the exterior. The most complicated 

 advance on nemerteans is seen in teleosteans, where the 

 lining layer of tissue round the ovary or testes becomes elon- 

 gated into a tube that conveys the products into the body- 

 cavity, and from the latter they pass through genital pores 

 to the exterior. Minor modifications on both types may 

 again take place. 



XIV. Development of the egg. 



Two types of embryonic development are known in 

 nemerteans, called respectively the direct and the indirect. 

 The former is typical of freshwater or land forms amongst 

 invertebrates, the latter develops amongst marine types. 

 The former occurs amongst protonemerteans, some meso- 

 nemerteans, and in all metanemerteans hitherto studied. 

 The latter is shown by some mesonemerteans and all he- 

 teronemerteans. Our knowledge regarding both modes 

 is still very fragmentary, but as holding the key to the 

 primitive development of vertebrates, should be most exact 

 and copious. It may be hoped then that fullest details 



