6 THE VOYAGE OF H.M.S. CHALLENGER. 



Diatoms vary greatly. These are generally more or less convex on the outside and 

 concave on the inside, increasing in this manner the capacity of the cell and at the same 

 time enabling it more readily to resist the external pressure which would otherwise tend 

 to crush it. Not only are the forms of the two valves in most cases symmetrical with 

 each other, but each valve is divided in many species into two similar portions by a 

 central rib, called a " raphe," which serves to strengthen the valve. 



In the centre of this raphe, and sometimes at its two extremities, swellings called 

 " noduli " often occur. The central nodulus, instead of being round, is sometimes extended 

 so as to form a kind of cross which is named a " stauros." 



Although in a great number of species a central raphe occurs, in many others no 

 such structure is to be found. Hence, upon the presence or absence of this raphe, 

 Professor H. L. Smith has proposed a new classification of Diatoms, distinguishing them 

 into Raphidiece, Pscudoraphidiew, and Cryptoraphidiew. It is to be remarked that this 

 classification has the defect of not being natural, and, moreover, it would seem that the 

 Cvyptorapliidicae should be called Araphidiem or Anaraphidiece, inasmuch as none of 

 the types included exhibit traces of the existence of a raphe or of any other analogous 

 arrangement. Nevertheless, since this system has the merit of simplicity, and has 

 already been adopted by the Austrian naturalist Albert Grunow, and by Dr Henri van 

 Heurck in his Synopsis des Diatome'es de Belgique, it will be followed in the present 

 Report. 



Diatoms, like all other living organisms, multiply and reproduce themselves. Multi- 

 plication, however, is not to be confounded with reproduction, but is to be understood as 

 an extension of individual life. Multiplication by duplication or division occurs in many 

 members of the vegetable kingdom ; on the other hand the process of reproduction is 

 common to every living thing. That multiplication by duplication is of frequent occur- 

 rence in Diatoms is proved by the fact that it has been already observed in forty or 

 more different species, yet it would appear that it ought to be regarded not as normal 

 but as exceptional. This process, which is also called fissiparous division or ternno- 

 genesis, takes place as follows: (1.) the nucleus or cytoblast becomes bisected ; (2.) a 

 central contraction of the internal protoplasmic mass takes place, and proceeds so far 

 that the latter becomes completely divided ; and (3.) a double wall or diaphragm, which 

 contemporaneously extends to the circumference in a direction normal to the ring or 

 cingulum, is formed. In this manner the primitive mother frustule gives origin to two 

 perfect daughter frustules, each of which is provided with an old and a new valve, the 

 latter being somewhat smaller in diameter than the former as its hoop is embraced by that 

 of the old valve. The process of duplication, as it progresses, must in this way lead to 

 a marked diminution of the diameter of the new frustules, and in a short time these would 

 become so small that they would be no longer characteristic of the species. In order, 

 however, to counteract this progressive diminution it is to be noted that the siliceous walls 



