54 INVEETEBRATA CHAP. 



I. HYDEOZOA 



TUBULARIA 



The type which we select for special descriptions is the common 

 hydroid Tubularia, species of which are abundant in shallower water 

 on British, Mediterranean, and American coasts. We base our 

 account on the careful work of Brauer (1891) who has worked out the 

 development of the Mediterranean species of Tubularia mesembryan- 

 themum. 



The British Tubularia indivisa is found attached to the bottoms 

 of old boats. The medusa in most species of this genus remains 

 permanently attached and the young pass through the earliest stages 

 of their development within the bell of the mother. In this respect, 

 as in the permanent attachment of the medusae, Tubularia is far 

 from exhibiting primitive or typical conditions, but the free- 

 swimming medusae of any particular species cannot always be 

 obtained, and the eggs of these can often only be reared through the 

 earlier stages of development. In many cases the hydroid stage is 

 unknown, and a picture of the complete development of a typical 

 hydroid which produces free-swimming medusae, can only be made 

 out by piecing together the only fragments of the life-histories of 

 many species which are known. It is for this reason that, in spite 

 of its manifest disadvantages, we choose Tubularia as a type. 



The egg and early segmentation stages are found by examining 

 transverse sections of the gonophores and by means of whole 

 mounts. The gouophores are the rudimentary medusae which remain 

 attached to the colony throughout life. On the mauubria of these 

 the eggs and sperm are produced. The eggs are dehisced into the 

 bell, in w T hich they undergo practically the whole of their develop- 

 ment, so that the bell is no longer a locomotor organ but a nursery. 

 The eggs are amoeboid and appear to segment fairly regularly, 

 but they are deformed by mutual pressure in the confined space in 

 which they find themselves. Only two or three are dehisced at one 

 time. 



The segmentation of the egg is somewhat irregular and leads, 

 after about sixteen segments have been formed, to the formation of a 

 hollow' vesicle or blastula. It is, however, a remarkable circumstance 

 that what appears to be an abnormal form of segmentation frecp^ently 

 occurs and leads also to the formation of a regular blastula. In this 

 latter form of development the nucleus divides repeatedly before any 

 division of the protoplasm occurs, and then subsequently the multi- 

 nucleate mass is cut into cells. When we find that these two methods 

 of development sometimes characterize different genera of the same 

 class of the animal kingdom, we are apt to think of them as very 

 different, but their occurrence side by side in the same species shows 

 that the physiological difference separating them must be very slight. 



The blastula stage is succeeded by a solid morula stage. This 



