The southward migration of the Aryan invaders does not appear to have extended, at the time when the hymns of the Rigveda were composed, much beyond the point where the united waters of the Panjab flow into the Indus. The ocean was probably known only from hearsay, for no mention is made of the numerous mouths of the Indus, and fishing, one of the main occupations on the banks of the Lower Indus at the present day, is quite ignored. The word for fish (matsya), indeed, only occurs once, though various kinds of animals, birds, and insects are so frequently mentioned. This accords with the character of the rivers of the Panjab and Eastern Kabulistan, which are poor in fish, while it contrasts with the intimate knowledge of fishing betrayed by the Yajurveda, which was composed when the Aryans had spread much farther to the east, and, doubtless, also to the south. The word which later is the regular name for "ocean" (sam-udra), seems therefore, in agreement with its etymological sense ("collection of waters"), to mean in the Rigveda only the lower course of the Indus, which, after receiving the waters of the Panjab, is so wide that a boat in mid-stream is invisible from the bank. It has been noted in recent times that the natives in this region speak of the river as the "sea of Sindh;" and indeed the word sindhu ("river") itself in several passages of the Rigveda has practically the sense of "sea." Metaphors such as would be used by a people familiar with the ocean are lacking in the Rigveda. All references to navigation point only to the crossing of rivers in boats impelled by oars, the main object being to reach the other bank (para). This action suggested a favourite figure, which remained familiar throughout Sanskrit literature. Thus one of the poets of the Rigveda invokes Agni with the words, "Take us across all woes and dangers as across the river (sindhu) in a boat;" and in the later literature one who has accomplished his purpose or mastered his subject is very frequently described as "having reached the farther shore" (paraga). The Atharva-veda, on the other hand, contains some passages showing that its composers were acquainted with the ocean.