392 



levski^ (1883) and Isclmochiton imitator as testified by Plate ^ (1897) 

 both species carry their young until they are in an advanced trochophore 

 stage. These to the number of about 200 in the case of Trachydermon 

 are held in the mantle cavity on each side of the foot. Plate speaks of 

 their extending over neighboring regions of the mantle and to some 

 extent upon the foot but in the present case this removal of the eggs 

 from the mantle cavity never occurs unless the animal has been disturbed. 

 It is interesting to note that while the gills are thus covered and respi- 

 ration impeded the proboscis (Mundscheibe) is highly distended with 

 blood and the lateral protuberances (Laterallappen) become much en- 

 larged and both may temporarily assume increased powers of respiration. 

 Nuttallina ihomasi which breeds during the mid summer carries its eggs 

 in two rod shaped masses, each with approximately 80 eggs, held in the 

 mantle cavity on each side of the foot. 



On several occasions I have removed some of the adults of T. ray- 

 mondi and N. thomasi to isolated tide pools and in these situations they 

 have bred and the young have lived until they became sexually mature. 

 Additional observations were made upon the growth of the last named 

 species in its native habitat and the results show that the conditions 

 just described are normal. As Mead'^ (1900) has shown for the starfish 

 [Asterias forbesii) and the soft-shell clam {3£ya arenaria] the growth of 

 the young may be surprisingly rapid if supplied with abundant nourish- 

 ment. If these conditions prevail both species of chitons attain sexual 

 maturity within one year. In the case of Tr. raymondi the adult may 

 be of maximum size (8 — 10 mm) but with Nut. thomasi there is usually 

 an increase of 3 mm during the early part of the second year, the largest 

 adults measuring on an average of 12 mm. 



The young of Kath. tunicata placed in tide pools nearer the mean 

 tide mark and therefore in conditions closely approaching those of their 

 native habitat may reach a total length of 25 mm in one year. The se- 

 cond year they may become sexually mature after having grown an addi- 

 tional 8 — 11 mm. During the third year they reach their average size, 

 55 mm. This rate of growth appears also to be characteristic of Mopalia 

 muscosa^ M. lignosa, Ischnochiton rcgidaris,IscJi. magdalenensis andlsch. 

 cooperi. In the case of Cryptochiton stelleri, which breeds some time in 

 February or March, the young in the early part of the following summer 



5 Kowalevski, A., Embryogénie du Chiton polii (Philippi) avec quelques 

 remarques sur le développement des autres Chitons. Ann. Mus. nat. Marseille. T. I. 



6 Plate, L. H., Die Anatomie und Phylogenie der Chitonen. Zool. Jahrb. 

 Suppl. 4—5 (Fauna Chilensis). 



■7 Mead , A. D., On the correlation between growth and food supply in starfish. 

 Amer. Nat., Vol. XXXIV. Observations on the soft-shell clam. Rept. of Commis- 

 sioners of Inland Fisheries, No. XIII. 



