98 TWENTY-FIRST REPORT, 



MATERIAL AND METHODS. 



The material for this work was obtained while studying at the Marine 

 Biological Laboratory, Woods Hole, during the summer of 1914. The shells 

 possessing Hydractiuia colonies were found at low tide in Eel Pond. In order 

 to prevent the polyps from contracting into abnormal shapes they were nar- 

 cotized by adding, drop by drop, a solution of ten per cent chloreton in absolute 

 alcohol till all power of contraction was lost. They were preserved in a four 

 per cent formalin solution. For histological structure the best results were 

 obtained with iron-hematoxylin stain. Sections were made from seven to 

 twelve micra in thickness. Camera lucicfa drawings were made of various 

 magnifications. 



THE POLYPS. 



The gasterozooids, as a rule, are the most numerous representatives of 

 the colony ; but sometimes during the summer months reproduction is so active 

 that the blastostyles occasionally are equally as numerous. These are the 

 longest of the polyps, often exceeding one-fourth inch in length. They possess 

 a conical hypostome, terminating in a large mouth. Around the base of the 

 hypostome are two circles of tentacles which increase in number with age 

 from ten to thirty. The longest tentacles that occur on members of the colony 

 are found here ; they are crowded with nematocysts. The external surface is 

 covered with a layer of ectoderm, which is continuous with the upper ectoderm 

 of the coenosarc. 



Since it is the function of this polyp to collect food for the entire com- 

 monwealth, it possesses the largest gastral cavity (PI. 1, Fig. 3). It is lined 

 with a single layer of endoderm, continuous with that of the endodermal canals 

 of the coenosarc. 



The ectoderm and endoderm are separated by a thin structureless layer 

 of mesogloea, which will not be referred to in description of the other polyps, 

 as it is common to all. 



The blastostyles are either male or female, though both sexes are never 

 found in the same colony. The mouth, and gastral cavity are both small. A 

 short distance below the mouth are two circles of tentacles varying in number 

 from ten to thirty, l)ut, unlike those of the gasterozooids, are very rudimentary, 

 consisting of knob-like structures crowded with nematocysts. Immediately 

 below the head the walls constrict into a narrow neck and then enlarge into 

 a globular dilatation from which arise the sporosacs. In both sexes the repro- 

 ductive cells arise in the body and migrate to the sporosacs. In female colonies 

 the sporosacs are filled with eggs that can be seen through the thin ectodermal 

 walls in the unstained, as well as stained, condition (PI. 1, Fig. 2). In the 

 male colonies the reproductive elements are small, very numerous, and stain 

 more deeply than the body cells (PI. 1, Fig. 4). Below the sporosacs the body 

 again narrows often into a slender thread. 



