TRANSACTIONS OF SECTION D, 655 



fragments and stones, and the sand-feeders, which further grind these up into fine 

 sand and mud. Both these series of forms are extremely scarce on the outer slopes 

 of atoll reefs, while they are the most abundant animals found within the lagoons 

 of our atolls. First the boring alga, Achyla, and the sponge, Cliona, penetrate the 

 living corals, extending into every septum and spine. They weaken the coral, and 

 80 riddle it that it is then easy for the boring Polychaeta, such as Pohjdora and 

 Eunice, to enter. Following these come -various Sipunculid worms, the bivalve 

 Lithodovius and the cirriped Lithotrija. A wave breaks off the coral mass, leaving 

 a bare surface, which more of the boring animals at once take advantage of. The 

 fallen coral mass is broken down into smaller and smaller fragments by the boring 

 animals. Then the sand-feeders come into action and grind up the coral frag- 

 ments into sand. Chief among these may be mentioned the sea-slugs, Holothuria 

 atra and Sfichopus chloronotus, which appear to retain within their guts the 

 coarser fragments of the sand, while the finer particles are swept out along their 

 ciliated grooves. Other forms are Sipunculns, numerous sea urchins, and Ptycho- 

 dera, the mound-like casts of the latter of which form a most conspicuous feature 

 of the landscape at low spring tides. Much of the finer material must pass into 

 suspension in the water, and be swept out of the atoll by the tidal and oceanic 

 currents, while the smaller the sand grains the greater the area they present for 

 solution. It will thus be clear what an important bearing the study of the boring 

 and sand-feedino; animals has on the formation of the lagoons of atolls. 



4, The Early Development of Muscles and Motor Nerves in Leindosiren. 



By J. Graham Kerr. 



Development of the Myotomes. — The conversion of the inner wall of the 

 myotome into a muscle-segment takes place in Lepidosiren in a manner closely 

 resembling what happens in Amphioxus and Petromyzon. I have been able to 

 make out the details of the process fairly completely. 



In early stages of development the inner wall of the myotome consists of a 

 simple layer of very large epithelial cells, columnar in transverse sections, but 

 really forming rectangular parallelepipeds somewliatcompressed in a dorso-ventral 

 direction and sloping inwards and downwards. The muscle-fibres appear as fine, 

 highly refracting, rod-like difl'erentiations of the protoplasm, running longitudinally 

 from end to end of the muscle-cell, and conspicuous by their staining deeply with 

 Heidenhain's iron hfematoxylin. These fibres become visible about stage 26, and 

 they are at first as a rule confined to the dorsal and ventral faces of the cells, 

 forming thus in each cell two layers, a dorsal and a ventral. Fibres also develop 

 between the outer ends of these layers, so that now the fibres in each cell show a 

 ID-shaped arrangement in transverse sections. 



As development proceeds other fibres arise in the protoplasm lying within 

 the limbs of the D-shaped arrangement, until the cell body is to a great extent 

 filled with the fibres. 



The mass of contractile fibres does not extend to the outer end of the cell. 

 This is occupied by clear protoplasm containing one or more large vacuoles. 

 Whether these contained glycogen in the fresh condition I cannot say. 



Up to stage 31 the outer wall of the myotome remains a simple layer of 

 cubical cells. About this period, however, active multiplication of the cells takes 

 place, and in transverse sections several layers of nuclei are seen. In horizontal 

 sections, also, numerous nuclei are seen scattered through the whole distance 

 between anterior and posterior boundarj' of the myotome. No cell limits are 

 visible, but we may take it possibly that the cell territory presided over by each 

 nucleus extends the whole length of the myotome, for now (stage 31 + ) striated 

 muscular fibrils begin to appear in the protoplasm between the nuclei, stretching 

 quite continuously the whole length of the myotome. In somewhat .shrunken 

 transverse sections each bundle of fibrils is seen to be contained in a sheath of 

 protoplasm containing a nucleus at one side, the whole forming a somewhat 

 cylindrical * cell ' stretching from the one myoseptum to the other. In life 



