CILETOPODA 303 



before the asexual zooid is entirely separated. This process 

 being repeated leads to the formation of chains of sexual 

 zooids attached to the original asexual stock, the youngest 

 member of the chain being that next to the proliferating 

 stock. Naturally all the zooids of one chain are of the same 

 sex. The formation of the cephalic appendages of the sexual 

 zooid is complete, but they are not exactly similar to those 

 of the asexual stock, nor are those of the males quite similar 

 to those of the females. 



Thus we have, in the Autolytinoe, sexual dimorphism, 

 which is exhibited in a slight degree in the Syllidinae, 

 distinctly developed in the sexual zooids. Sexual dimorph- 

 ism is here associated with alternation of generations, or, 

 as it is technically termed, metagenesis, but, as I have 

 explained above, the latter phenomenon is easily explained 

 as resulting from the familiar process of recrescence, or new 

 growth of lost parts. I have now to describe the sexual 

 dimorphism in some detail in certain examples. 



As in many other cases of sexual polymorphism, the three 

 forms in the Autolytinse were originally described as distinct 

 genera. The asexual form was Autolytus, the male sexual 

 zooid Polybostrichus, and the female Sacconereis. In all 

 the Syllidae the embryos are carried by the female attached 

 to the skin, and in many cases, as in the Autolytinse, they 

 are contained in a sort of oval pouch attached to the ventral 

 side of the mother. This ovigerous sac is stated to be formed 

 by the secretion of epidermic mucous glands. 



The connection of the three forms was first proved by 

 Alexander Agassiz in 1863, 1 who studied a species on the 

 American coast named Autolytus comutus. In the asexual 

 stock the head has three tentacles, the median being the 

 longest (palps are rudimentary in the sub-family). The 

 segment next the head has two pairs of cirri, one long and one 

 1 Boston Journ. Nat. Hist., 1863, "The Embryology of Autolytus cornutus." 



