46 BULLETIN OF THE BUREAU OF FISHERIES 



hydroid medusa Mitrocoma cruciata reaches maturity during this same month, 

 when it may appear near shore in numbers sufficient to give a distinctive aspect to 

 the tow, as was the case at the mouth of Penobscot Bay on June 14, 1915 (station 

 10287 p. 348). For the sake of clarity I should point out, at the risk of repetition 

 (p. 389), that diatoms still swarm along a narrow coastwise belt east of Penobscot 

 Bay in June. 



The advance of summer (from June on) sees an actual decrease in the number of 

 copepods, owing, no doubt, to the destruction wrought among them by fishes and 

 other enemies (p. 97). In part this decrease is made good by constant reproduction, 

 evidence of which was afforded by an abundance of copepod nauplii near Cape Cod 

 on July 8, 1913 (station 10057, surface), on July 7, 1915 (station 10300), and on 

 August 29, 1916 (station 10398) ; likewise by the presence of large numbers of juvenile 

 Calanus 23 between Cape Ann and the Isles of Shoals in July, 1912. The offshore 

 banks also serve as a copepod nursery in July — at least locally — for copepod eggs, 

 nauplii, and juveniles abounded on the surface near Nantucket Lightship on the 

 25th of that month in 1916 (station 10355), while the presence of young Calanus 

 at various stages in development in most of the summer towings proves that this 

 copepod breeds more or less regularly throughout the summer. Our experience, 

 however, does not suggest that sufficient reproduction takes place during the warm 

 months to maintain the local stock of calanoid copepods against depletion by the 

 many dangers to which it is subjected. 



As copepods dwindle in numbers the other groups of common boreal animals 

 increase, lending an increasing diversity to the plankton of the offshore parts of the 

 gulf during the summer, most noticeably in the western side, where the plankton 

 is most monotonously calanoid in May and June, thus producing the midsummer 

 state already described (p. 17). Events notable in this gradual alteration are a 

 great production of Euthemisto, resulting from local centers of reproduction such 

 as I have just mentioned (p. 20) ; the active propagation of euphausiids (p. 20) ; a 

 general penetration toward the western and northwestern shores of the Gulf on the 

 part of the pteropod Limacina retroversa (p. 119); the appearance of shoals of the white 

 and red jellyfishes (Aurelia and Cyanea) in the coastal belt as they disperse and 

 drift seaward from their estuarine nurseries (pp. 360, 362) ; the presence of large Stauro- 

 phora, often in abundance (p. 342) ; and the offshore swarming of the hydroid medusa 

 Phialidium languidum (p. 350). It is during the summer, too, that the large and 

 conspicuous arrow-worm Sagitta serratodentata first appears in any number in the 

 gulf as a visitor from warmer waters to the south and east outside the edge of the 

 continent, and spreads its range northward and westward as described elsewhere 

 (p. 322). The copepod population, also, becomes diversified as the summer advance 

 by increasing numbers of Anomalocera and Centropages, not only within the gulf 

 but also on Georges Bank, where the former (which we did not find in spring) is 

 practically universal and comparatively abundant in August. 24 The ctenophore 

 Pleurobrachia pileus reaches its maximum abundance on the German Bank ground 



" Identified by Dr. C. O. Esterly. 



'< The "green copepod" of Doctor Kendall's field notes. 



