288 



ROBERT TRACY JACKSON ON THE 



wire and suspended between stakes. The plates were placed in pairs with the edges 

 apposed so that the centres vv^ere widely separated and chambered pockets wei-e thus 

 formed. The}^ were fairly successful. In 1888 when the pond was continnally fidl the 

 plates worked more successfully, the oysters growing with extreme rapidity. Tlie 

 rapid growth was probably due to the fact that the oysters were suspended and there- 

 fore kept very clean and also to the increased warmth of the water in the shallow pond 

 as compared with water in the open bay. In the chambers formed by the apposed plates, 

 similar forms to those found in the di-ain pipes were attached or entrapped. The value 

 of obtaining fixed forms like oystei'S on thin wood is that they are easily removed for 

 studv or may be decalcified and sectioned without removal. Should wooden plates be 

 desired, it is best to secure those which are cut from the block in a dish-forui, not those 

 which are cut flat and then j^i'essed into a dish-form; because the latter flatten out when 

 exposed in Avatei-, whereas the former retain their curved shape. 



Cocaine was found a most valuable reagent in paralyzing Pelecypods which it was 

 wished to kill distended or to dissect alive. A four per cent solution was used and was 

 added gradually to a small volume of sea-water containing tlie animal. By this means 

 only was I successful in killing young Pectens with the byssal attachment intact. Ad- 

 ditional material was collected at various localities near Boston and some was loaned me 

 by museums as noted in the text. 



III. Classification of Stages of Gkowtii and Decline. 



In successive stages of growth of the individual, ])eculiai'ities are often found which 

 are characteristic of definite periods. These periodical chai'acters have been studied, 

 especially in their later phases, by Professor Hyatt in fossil Cephalopods, and as a con- 

 venience in considering the several periods, he has (33-4) devised a nomenclature of the 

 stages of growth and decline,-" which he believes is' applicable throughout the animal 

 kingdom.^ 



We will now consider this elassificatiou in its application to the mollusca. Tiie no- 

 menclature devised for early embryonic stages consists of names very broadly recalling 

 the grand divisions of the animal kingdom. They are protembiyo, mesembi-j^o and met- 

 embryo. The following names, neoembryo and typembryo, are applied to later stages 

 of embryonic development. 



Professor Hyatt considers that the neoembryo mollusc is the trochosphere stage, and 

 that it has not yet acquired charactei's peculiar to a mollusc. The trochosphere stage 

 (PI. XXIII, figs. 3-4), before the formation of the shell-gland, certainly has not yet ac- 

 quired characters which would define it as a mollusc, and we would limit the neoembiyo 

 stage to that jjeriod of develoi^ment.^ A comparison of this stage in molluscs with Pro- 



' Besides tbe references cited, tliis paper was published 

 in tlie American Naturalist October, 1888. An abstract 

 was pul)lished in Science, Vol. xi, No. 260. 



- Professor Hyatt says that his nomenclature is similar 

 to that adopted by Professor Haeckel; but is also supple- 

 mentary, and based on morphological, rather than on phys- 

 iological grounds. In his jiaper he discusses the relations 

 of the two systems of nomenclature. 



^ The ideal trochosphere of Professor Lankester (4-i) as 

 that author says : "... exactly agrees with the lar- 

 val form of many Cha^topod worms and other Cojlomata ; 

 most remarkable is its agreement with the adult form of 

 the wheel animalcules or Rotifera . . . ." Again he 

 says : "So far the young mollusc has not reached a defi- 

 nitely molluscan stage of development, being only in a con- 

 dition common to it and 'other Ctelomata.' " Some of 



