290 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



Clark observed tlie egp:s being ejected in a sort of stream from the 

 openings into the water and settling in loose clusters on adjacent objects. 

 Yerrill and Carpenter have conlirmed these observations, and add that 

 the hinder part of the foot is so raised as to form a sort of funnel, out of 

 which the eggs emerge. But these eggs, in some cases at least, are 

 already imi)regnated, and somevfhat far advanced in developement before 

 they leave the oviduct. I have myself observed, in several individuals^ 

 Avhich had been some years in spirit, eggs developed as far as the first 

 utage figured (47fl), which had never left the ovisac. In this connection 

 it may be observed that, if the sperma be ejected into the water, there 

 is nothing to prevent the spermatozoids from entering the wide aper- 

 tures of the fenestrne (in some species at least), and thus impregnating 

 the eggs in the ovary. In this way may be explained the presence of 

 spermatozoa in the female Chiton PaUasii noted by Middendorf. In 

 some species with very small ovarian openings, this internal impregna 

 tion would be attended with more difficulty. In some species, a large 

 so-called ''slime gland" is present, lying under the middle line of the 

 ovisac. Gray states that in some Chitoms the egg is enfolded in a thick, 

 vesicular, folded envelope, but I did not notice anything of the kind in 

 those I examined. I observed no microi)yle, but having only specimens 

 hardened hj long immersion in spirits these observations cannot be 

 deemed conclusive. In all the species specially examined to determine 

 the character of the ova, the eggs were spherical, with a rather tough 

 skin, quite smooth, with no trace of lime in it, and apparently in no 

 way attached to the walls of the ovary when ripe.* 



BeveJopement. — "Nothing later than the brief but admirable researches 

 of Loven, now thirty years old, has come to hand. His figures are here 

 reproduced, with a summary of what has been observed, to stimulate 

 further enquiry in tliose favorably situated. 



The Chitons difier from most Mollusks in that the shell does not appear 

 on the embryo until some time after they are hatched. In this connec- 

 tion, the observations of Krohn on MarsenUdw may be referred to. 



The embryo of Chiton dnercus is oval, with no trace of shelly valves 

 Ot deiu'essions for them, and is divided into two nearly equal parts by a 

 transverse depression, the margins of which are ciliated. On the middle 

 of the up])er part is a tuft of filaments which move slightly. At each 

 end of the depression are two dark points, representing the eyes. 



The yonng when hatched (Fig. 47 h, c) become more elongated, tlie 

 iront part is finely ciliated, aiul the tuft occasionally vibrates. The 

 hinder part extends more rapidly and becomes conic. The back is 

 marked by seven furrows ; between these the first rudiments of the shelly 



* Dr. V. Iheriiig describes the egg of C. squamosus as covered ^vitll peculiar thorns, 

 five-eided solid cohimns, expanding at tlie distal end into a cnp whose edge is cut into 

 five iioiuts. C. Cajctamta and fnscicularls had eggs (tovered with a grooved and irregu- 

 larly furrowed niembrane, as described by Gray, but without thorns. The vesicular 

 membrane thus may be considered as a chorion. In tho immature stages, the eggs are 

 enclosed in follicles of the tissues of tho ovisac. 



